Elisse, Interesting info I had not heard of this site. I had a look at it and can find no way of adjusting the size. 96x64 which is the resolution being converted to is the default size originally for 1.2 inch players.
I have read of some mac owners using Parallel Desktop and being able to run the PC AMV converter. You can download a 15 day trial
HERE but then you would also need to obtain a copy of Windows XP so may not be much help to you.