Set non-power-of-2 Max Size values for textures/sprites in Unity

CustomMaxSizeSetter.cs

using UnityEditor;
using UnityEngine;

public class CustomMaxSizeSetter : EditorWindow
{
	private const int MINIMUM_MAX_SIZE = 32;
	private const int MAXIMUM_MAX_SIZE = 2048;

	private int initialMaxSize = -1;
	private int currentMaxSize = -1;

	private Texture2D[] selection;
	private string selectionLabel;

	[MenuItem( "Window/Custom Max Size Setter" )]
	private static void Init()
	{
		CustomMaxSizeSetter window = GetWindow<CustomMaxSizeSetter>();
		window.minSize = new Vector2( 250f, 85f );
		window.titleContent = new GUIContent( "Custom Max Size" );
		window.Show();
	}

	private void OnEnable()
	{
		OnSelectionChange();
	}

	private void OnGUI()
	{
		GUILayout.Label( selectionLabel, EditorStyles.boldLabel );

		EditorGUI.BeginDisabledGroup( selection == null || selection.Length == 0 );

		EditorGUI.showMixedValue = currentMaxSize < 0;
		EditorGUI.BeginChangeCheck();
		int maxSize = EditorGUILayout.IntSlider( currentMaxSize, MINIMUM_MAX_SIZE, MAXIMUM_MAX_SIZE );
		if( EditorGUI.EndChangeCheck() ) // Otherwise, IntSlider clamps value from -1 to MINIMUM_MAX_SIZE with no user input
			currentMaxSize = maxSize;
		EditorGUI.showMixedValue = false;

		EditorGUILayout.Space();

		EditorGUI.BeginDisabledGroup( currentMaxSize < 0 || initialMaxSize == currentMaxSize );

		if( GUILayout.Button( "Apply" ) )
		{
			AssetDatabase.StartAssetEditing(); // Apart from batching the reimport operations, this also ensures OnProjectChange isn't called in the middle of this for-loop
			try
			{
				for( int i = 0; i < selection.Length; i++ )
					SetMaxSizeOfTexture( selection[i], currentMaxSize );
			}
			finally
			{
				AssetDatabase.StopAssetEditing();
			}
		}

		EditorGUI.EndDisabledGroup();
		EditorGUI.EndDisabledGroup();
	}

	private void OnProjectChange() // Texture Max Size might be changed from the Inspector and etc.
	{
		OnSelectionChange();
	}

	private void OnSelectionChange()
	{
		selection = Selection.GetFiltered<Texture2D>( SelectionMode.Assets );
		if( selection == null || selection.Length == 0 )
		{
			selectionLabel = "Max Size of \"Nothing Selected\":";
			initialMaxSize = currentMaxSize = -1;
		}
		else if( selection.Length == 1 )
		{
			selectionLabel = "Max Size of \"" + selection[0].name + "\":";
			initialMaxSize = currentMaxSize = GetMaxSizeOfTexture( selection[0] );
		}
		else
		{
			selectionLabel = "Max Size of \"" + selection[0].name + "\" and " + ( selection.Length - 1 ) + " more:";
			initialMaxSize = currentMaxSize = GetMaxSizeOfTexture( selection[0] );
			for( int i = 1; i < selection.Length; i++ )
			{
				int maxSize = GetMaxSizeOfTexture( selection[i] );
				if( maxSize != initialMaxSize )
				{
					initialMaxSize = currentMaxSize = -1;
					break;
				}
			}
		}

		Repaint();
	}

	private int GetMaxSizeOfTexture( Texture2D texture )
	{
		TextureImporter textureImporter = (TextureImporter) AssetImporter.GetAtPath( AssetDatabase.GetAssetPath( texture ) );
		return textureImporter.maxTextureSize;
	}

	private void SetMaxSizeOfTexture( Texture2D texture, int maxSize )
	{
		TextureImporter textureImporter = (TextureImporter) AssetImporter.GetAtPath( AssetDatabase.GetAssetPath( texture ) );
		textureImporter.maxTextureSize = maxSize;
		textureImporter.npotScale = TextureImporterNPOTScale.None;
		textureImporter.SaveAndReimport();
	}
}

How To

Simply create a folder called Editor inside your Project window and add this script inside it. Then, open Window-Custom Max Size Setter, select one or more textures and adjust their Max Size values.

Since NPOT texture sizes don't work well with compression algorithms, this technique is mostly useful for fine-tuning the max sizes of sprites that are packed in Sprite Atlases.