I'm wondering how much bigger the file size will be for an ipad specific game vs the same game made for iPhone/iPod. This can help determine which ipad to get. Thanks.
The size difference in the same apps for each will be so small it won't affect your choice. Even if they completely re-do all the graphics, it won't change the the size by a huge amount.
Size-wise, we're not normally talking about Gigabytes of data. The largest app I've personally heard of is Navigon and it's 1.44 GB. I think even the largest games are only a few hundred Megabytes currently. Scaling up the textures in a FPS isn't going take tons of space comparatively.
It shouldn't be too much, but I reckon it will be a noticeable amount, I upgraded one storage level just in case.