What would be a sensible maximum texture size to expect on today's hardware, factoring in the whole range of graphics cards, integrated graphics and legacy systems that could be realistically be expected to be still in use?

I am playing around with some 2D prototypes and am currently making tile sheets to load into an OpenGL texture. I would like to be able to load as much as possible into a single texture, which currently isn't going to be a problem, however it would be nice to know for the future what I should peg as a limit for the maximum size for a tile sheet so I can plan the scope of the available tiles accordingly.

If you want to support tablets, you should go no higher than 20482, which is what Tegra 3 supports (iPad3/Tegra 4 supports 40962). If iPad, Nexus, or Galaxy Tab are possible candidates, don't go over 20482.

For desktop, 40962 is a reasonable number if you want to account for really old graphic cards (5-8 years) as well, though almost all of them could do 81922 even back then. For reasonably recent graphics cards, 81922 is a mostly OK assumption.

When in doubt, Lorenzo Gatti's argument holds true: Ask yourself if they really should be supported. Not only will your game probably look and feel like crap on hardware that doesn't meet some minimum specs (this gives a bad impression), but also if you plan to sell your game it is highly unlikely that you'll be getting paid. Someone who can't (or doesn't want to) put 20€ into a graphics card can't afford (or doesn't want) to pay you either.

OpenGL 4.1 mandates 163842, so if you design primarily for that version, you can blindly use this size without asking. If it takes another year or so before your're finished, the share of cards not capable of doing OpenGL 4.1 will be even smaller.

Thanks to both of you for your replies, that's just what I needed - a sanity check for myself that expecting 2048x2048 texture support is a reasonable requirement.