if you dimensionally scale - or DIM - your images instead of physically scaling them theres a much broader range of sizes you can go to - thats how billboard size scans are created. theres a big difference between something being technically accurate and optically accurate - you can get away with a lot depending on the end result you are after.
The "rule of thumb" is that it's best to allow 200 pixels per inch when printing, so 3000x2000 would print at 15"x10". However, this is for looking at closely - the bigger you print your picture the further away people are going to view it from. If you look at a billboard close up you can see a lot of "defects", but from the distance it's supposed to be viewed it looks fine. I'm not saying you'd get away with printing at billboard size, but you can go a lot bigger than you might think and get perfectly acceptable results.
In the camera shop where I get my photos made there is a series of prints from 4x5 all the way up to 4' x 5'. This image was shot with a 6mpx Canon Rebel. It is only in the largest size that you can see artifacting, and that is only if you look closely. I'd experiment, and see where it happens. There are inexpensive on-line services where you can find out for a small investment.
thanks