Hi vmxmr,
Sorry for the hassle, and many thanks for posting back with your detailed findings.
I can reproduce this behavior here too (Fusion 5.0.3 on OS X 10.8.3). It's definitely a UI bug, and it occurs just as you describe: If you change the memory size and then immediately close the window without doing anything else, the change to the memory size doesn't get saved. We now have a fix prepared for this issue internally, although I can't say when it will make its way into a public release of VMware Fusion. Until then, hopefully the workaround you provided will suffice.
Cheers,
--
Darius