For non square mapblocks try specifying the full dimesion, when you use a single value like '1' or '2' it defaults to a square size (10x10, 20x20). And yes a '3' would be 30x30.
For a 30x20 you can use: size: [3,2,0]
For a 30x10: size: [3,1,0]
For a 20x10: size: [2,1,0]
For a 10x20: size: [1,2,0]
As far as I can tell, even tho the resize command is the only one that uses the 3rd value (height) it must still be listed for other commands when using it in this format, even if it's just a 0.