Automator Service that receives Image files, asks for a width and resizes the image to that width, and resizes a copy of the original to the double of the entered width, saving this with a @2x sufix. Uses Acorn. Applescript appended for reading, .workflow appended for usage (open to install).

Applescript code.applescript

property enteredWidth : 0
property newPath : ""

on run {input, parameters}
    repeat with fileAlias in input
		try
			display dialog "Enter width for 1x of " & (POSIX path of fileAlias) & ":" default answer ""
			set dialogInfo to result
			set enteredWidth to (text returned of dialogInfo as integer)
			set the newPath to my replace_chars((POSIX path of fileAlias), ".png", "@2x.png")
		on error dialogError
			return
		end try
		
		tell application "Acorn"
			set doc to open (POSIX path of fileAlias)
			tell doc
				trim
				duplicate
				resize image width enteredWidth
				save
				close
			end tell
			tell front document
				resize image width (2 * enteredWidth)
				save in newPath
				close
			end tell
			
		end tell
	end repeat
	return input
end run



on replace_chars(this_text, search_string, replacement_string)
	set AppleScript's text item delimiters to the search_string
	set the item_list to every text item of this_text
	set AppleScript's text item delimiters to the replacement_string
	set this_text to the item_list as string
	set AppleScript's text item delimiters to ""
	return this_text
end replace_chars

Resize for 1x and 2x.workflow

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>AMApplicationBuild</key>
	<string>346</string>
	<key>AMApplicationVersion</key>
	<string>2.3</string>
	<key>AMDocumentVersion</key>
	<string>2</string>
	<key>actions</key>
	<array>
		<dict>
			<key>action</key>
			<dict>
				<key>AMAccepts</key>
				<dict>
					<key>Container</key>
					<string>List</string>
					<key>Optional</key>
					<true/>
					<key>Types</key>
					<array>
						<string>com.apple.applescript.object</string>
					</array>
				</dict>
				<key>AMActionVersion</key>
				<string>1.0.2</string>
				<key>AMApplication</key>
				<array>
					<string>Automator</string>
				</array>
				<key>AMParameterProperties</key>
				<dict>
					<key>source</key>
					<dict/>
				</dict>
				<key>AMProvides</key>
				<dict>
					<key>Container</key>
					<string>List</string>
					<key>Types</key>
					<array>
						<string>com.apple.applescript.object</string>
					</array>
				</dict>
				<key>ActionBundlePath</key>
				<string>/System/Library/Automator/Run AppleScript.action</string>
				<key>ActionName</key>
				<string>Run AppleScript</string>
				<key>ActionParameters</key>
				<dict>
					<key>source</key>
					<string>property enteredWidth : 0
property newPath : ""

on run {input, parameters}
	repeat with fileAlias in input
		try
			display dialog "Enter width for 1x of " &amp; (POSIX path of fileAlias) &amp; ":" default answer ""
			set dialogInfo to result
			set enteredWidth to (text returned of dialogInfo as integer)
			set the newPath to my replace_chars((POSIX path of fileAlias), ".png", "@2x.png")
		on error dialogError
			return
		end try
		
		tell application "Acorn"
			set doc to open (POSIX path of fileAlias)
			tell doc
				trim
				duplicate
				resize image width enteredWidth
				save
				close
			end tell
			tell front document
				resize image width (2 * enteredWidth)
				save in newPath
				close
			end tell
			
		end tell
	end repeat
	return input
end run



on replace_chars(this_text, search_string, replacement_string)
	set AppleScript's text item delimiters to the search_string
	set the item_list to every text item of this_text
	set AppleScript's text item delimiters to the replacement_string
	set this_text to the item_list as string
	set AppleScript's text item delimiters to ""
	return this_text
end replace_chars

</string>
				</dict>
				<key>BundleIdentifier</key>
				<string>com.apple.Automator.RunScript</string>
				<key>CFBundleVersion</key>
				<string>1.0.2</string>
				<key>CanShowSelectedItemsWhenRun</key>
				<false/>
				<key>CanShowWhenRun</key>
				<true/>
				<key>Category</key>
				<array>
					<string>AMCategoryUtilities</string>
				</array>
				<key>Class Name</key>
				<string>RunScriptAction</string>
				<key>InputUUID</key>
				<string>FA71A531-2716-4087-86C3-1A294B5006CE</string>
				<key>Keywords</key>
				<array>
					<string>Run</string>
				</array>
				<key>OutputUUID</key>
				<string>A987B4EB-BA88-4E2C-B272-9A20E19E9B64</string>
				<key>UUID</key>
				<string>1BB04C1D-F56B-4AFD-8796-DE092148071B</string>
				<key>UnlocalizedApplications</key>
				<array>
					<string>Automator</string>
				</array>
				<key>arguments</key>
				<dict>
					<key>0</key>
					<dict>
						<key>default value</key>
						<string>on run {input, parameters}
	
	(* Your script goes here *)
	
	return input
end run</string>
						<key>name</key>
						<string>source</string>
						<key>required</key>
						<string>0</string>
						<key>type</key>
						<string>0</string>
						<key>uuid</key>
						<string>0</string>
					</dict>
				</dict>
				<key>isViewVisible</key>
				<true/>
				<key>location</key>
				<string>473.000000:748.000000</string>
				<key>nibPath</key>
				<string>/System/Library/Automator/Run AppleScript.action/Contents/Resources/English.lproj/main.nib</string>
			</dict>
			<key>isViewVisible</key>
			<true/>
		</dict>
	</array>
	<key>connectors</key>
	<dict/>
	<key>workflowMetaData</key>
	<dict>
		<key>serviceApplicationBundleID</key>
		<string>com.apple.finder</string>
		<key>serviceApplicationPath</key>
		<string>/System/Library/CoreServices/Finder.app</string>
		<key>serviceInputTypeIdentifier</key>
		<string>com.apple.Automator.fileSystemObject.image</string>
		<key>serviceOutputTypeIdentifier</key>
		<string>com.apple.Automator.nothing</string>
		<key>serviceProcessesInput</key>
		<integer>0</integer>
		<key>workflowTypeIdentifier</key>
		<string>com.apple.Automator.servicesMenu</string>
	</dict>
</dict>
</plist>