Install AutoHotKey from www.ahkscript.org and copy the below code into a text file and save it as BulkFilesizeRename.AHK (or whatever you want to call it).
FileSelectFolder,FolderName
; Remove the below semicolon if you want it to recurse through all folders inside the selected root folder and rename the folders in turn
Loop,%FolderName%\*.*,2 ;,1
{
TempFolder = %A_LoopFileFullPath%
FolderSize = 0
Loop, %TempFolder%\*.*, 1 ;,1
{
FolderSize += %A_LoopFileSizeMB%
}
FileMoveDir,%TempFolder%,%TempFolder% - %FolderSize% MB
}
Pick the root folder where the other folders are stored, and it'll do its thing. Remove the semicolon from the first Loop and you'll be able to rename the folders recursively - each folder inside the folder you pick, and the folders inside those, will be named accordingly to their total file size, all the way down. The folder you choose from the prompt won't have its name changed.