The only real system effect of making the maps larger and/or more complex is RAM requirement. The more stamps added, and the larger the maps are made, the more RAM is needed to load the map quickly. Of course, it'll load eventually - it'll just take longer. CPU speed can be a bit of an issue too, but you'll probably want to stamp a G3+ logo on the game anyway, as below that it gets a little too slow, I think. Nowadays, anyway.
There's no real way to figure it out though - I suppose the best you can do is just add up the size requirements of all your stamps and think about how much RAM is needed to load them all into. I don't think there's any way to guess with the RAM that the map will require - if you can drop into OS9, you could do a test though. Create a map with a predetermined number of stamps (ie: figure out the size of them all) and say, 100x100 in size. Load up the map in OS9 in a window and go Apple > About this Computer and see how much RAM the game is using. Replace it with a new map that's 500x500 with the exact same stamps and see how much it changes.
Of course, the flaw here is that it's in OS9, and if you're developing for the OSX market, the RAM usage is different, especially in application allocation of RAM. Might give you some rough ideas at least - and do keep in mind that when running OSX, the system requires around 64MB of RAM to page through, and that Classic pushes that to 128MB - hence the minimum requirements for the system.
Eh. I'm rambling. Something to consider, perhaps.
-Andiyar
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"Any good that I may do here, let me do now, for I may not pass this way again"