I use a 13" Air with a 6.75x4.5" intuos4 (8" diagonal).
I also plug that Mac into a 24" external monitor and use it with a bluetooth keyboard and an 8.25x6.5" intuos3 (10.5" diagonal). Maybe the same model tablet you have - PTZ-630? I don't think using your old tablet is likely to be a problem.
The only way I can really see it being a problem is if your tablet is very definitely not widescreen and your new screen is. They you'll either have to have a dead area across the top and/or bottom of the tablet, or uncheck the "force proportions" box in the mapping tab of the Wacom prefs, which is only great if you like moving your hand in a circle and getting an oval.
ALSO, a word of advice: if you decide to move up from the stand that comes with your monitor to an adjustable arm, spend the $50 or so extra to get a spring-loaded/gas-powered/otherwise-assisted arm. It makes it a LOT easier to move the monitor around to deal with different work postures, or turn it to portrait mode. Oh wait you're talking about moving from a laptop to an iMac aren't you? Maybe never mind this paragraph. Though apparently a VESA mount is a built-to-order option for iMacs, which could be pretty cool.