Photo enlargement recommendations...

About a week ago, I shot a horse show for a friend of mine, just for fun. Her birthday is coming up soon, and I've decided to get her an enlargement of one of the pics I took.

I took it with a Sigma 70-300 DG APO on an XTI, so it's not extremely sharp or anything. I have absolutely no experience with printing/enlarging, but the image looks quite sharp up to about 50%, then steadily loses sharpness approaching 100% I would imagine this makes sense, but I don't know if the picture is sharp enough for a poster-size print.

My question is how large I should go for a print. I don't know how much Costco process the image for an enlargement. I chose Costco because it's cheap, quick, and appears to have a good reputation.

I'm thinking a 20x30 might be too large; it might not be sharp/detailed enough.

The shot is of her on her horse, and there are some people in the background. Ideally, I would like to photoshop the people out of the image. Having no experience with printing, though, I don't know if that would be extremely conspicuous with an enlargement.

Anyone care to weigh in?

If you haven't cropped the image very much, then you should have no problem at all printing 20x30. Keep in mind that the viewing distance of a 20x30 print is between four and five feet so if you inspect the print from six inches you're not going to be happy

You might not get 70-200f4L IS sharp but you should be able to get pretty darned sharp with this combo certainly more than acceptable to look at past 50% zoomed in. Were you shooting from a tripod?

Kurt

Nope, handheld. It was at a horse show, so I needed to move around and change focal length and whatnot, so a tripod wouldn't have worked.

Either way, I think I'll go for the full 20x30... thanks!

If you really want to know what the print will look like then calculate out the ppi you will get with a 20x30 print and then make an 8x12 crop from the center. That should show you exactly how it will turn out for only a few dollars.