As alexh said, keeping it 1:1 similar to the original form factor will open up a LOT of other uses for your project and you could start gathering for interest in those other niches too.
Having a product which is directed to many different uses will certainly raise the orders, which will in turn lower the price for everyone. It's a Win-Win situation!
Anyway, keep in mind that there are cheaper alternatives around for expanding an A600, and most Amigans will probably go for those. PCMCIA SRAM might've been a great idea back in the day, but nowadays with cheap internal expansions readily available its use cases have been reduced greatly.
Just my 2c
