Here's a good site for resolution tips, (don't be misled by "scan", it covers all equipment issues.) I just bought the book and I'm working my way through it.
Great information on resolution.
Colin gave good advice. Resolution is very image specific and output specific, you should experiment. It surprised me that the difference between 150 - 300 ppi wasn't that noticeable to my eye.
To start with a "basic rule of thumb" it would be divide the pixels by the printing resolution desired to get "optimum inches."
example 2500 divided by 300 = 8.33"
1900 divided by 300 = 6.33"
There's a lot of other options like "upsampling" etc. but, the key factor is to experiment with your particular situation because you truly will be surprised by the different factors.
Guy, keep in mind that I'm new at this stuff, so, could be I know just enough to be "dangerous." (translated--give bad advice.)