This simple Artifactory plugin does a job of Calculating repository size by recursively iterating over Repository items and adding sizes of the artifacts.

RepositorySize.groovy

import org.artifactory.repo.RepoPathFactory

def getDirectorySizeRecursively(def repoPath){
  def children = repositories.getChildren(repoPath)
	if (children.size() == 0) return 0
	
	def total = 0
	children.each { item ->
		if (item.folder) {
			total += getDirectorySizeRecursively(item.repoPath)
		} else {
			total += item.size
		}
	}
	total
}

executions {
	/**
	 * This method expects parameter repoName to be passed. It will than calculate the TOTAL size of repository and return the value.
	 * If the repo name is invalid or not provided it will fail.
	 * Sample invokation:
	 * http://localhost:8081/artifactory/api/plugins/execute/getRepositorySize?params=repoName=ext-release-local
	 */
	getRepositorySize() { params ->
		
		if (params.size() > 0 && params['repoName'] && params['repoName'].size() > 0){
			def repoName = params['repoName'][0]
			if (repositories.localRepositories.contains(repoName)){
				message = getDirectorySizeRecursively(RepoPathFactory.create(repoName, '/'))
				status = 200
			} else {
				message = "Repository $repoName doesn't exists"
				status = 404
			}
		} else {
			message = 'Missing repository name parameter repoName={name}'
			status = 400
		}
	}

	/**
	 * Prepare report on all repositories
	 * Sample invokation:
	 * http://localhost:8081/artifactory/api/plugins/execute/getAllRepositorySizes
	 */
	getAllRepositorySizes() { params ->
		def repositoryReport = "[\n"
		def totalNumberOfRepositories = repositories.localRepositories.size()

		repositories.localRepositories.eachWithIndex { repoName, index ->
			def repoPath = RepoPathFactory.create(repoName, '/')
			def size = getDirectorySizeRecursively(repoPath)

			repositoryReport+="{\"repoName\":\"$repoName\", \"sizeInBytes\": $size}"
			if (index < totalNumberOfRepositories -1) {
				repositoryReport+=",\n"
			}
		}

		repositoryReport+="\n]"
		message = repositoryReport
		status = 200
	}
}

sample-response.json

[
{"repoName":"libs-release-local", "sizeInBytes": 122233},
{"repoName":"libs-snapshot-local", "sizeInBytes": 0},
{"repoName":"plugins-release-local", "sizeInBytes": 0},
{"repoName":"plugins-snapshot-local", "sizeInBytes": 0},
{"repoName":"ext-release-local", "sizeInBytes": 122233},
{"repoName":"ext-snapshot-local", "sizeInBytes": 0}
]

This is a great one!
Care to contribute it to https://github.com/JFrogDev/artifactory-user-plugins?