Sharpening for large canvas or metal prints?

Just got back from a great tour of Arches NP, Canyonlands NP, and Monument Valley. I have some shots I'd like to make into 20x30 prints, maybe gallery wrap or on metal. I've searched but haven't found much help so hopefully someone can chime in who does this a lot. All photos were shot with the 5D MkIII. I'm thinking of using Pro DPI to do all the printing as they have received some very good reviews.

1 - Should I re-size these to the actual print size before sending it or let them do it?

2 - Should I sharpen before or after I re-size or again, let them do it?

3 - Sharpening for large prints - I imagine it would be a lot stronger than sharpening for an 8x12 or 16x20. I've got Nik's Output Sharpener and it seems to do some amazing things. Just not sure on settings to try.

Thanks!
James

1. Leave your images at their native resolution and let them upsample, unless you have software specifically designed to do that task for you. Photoshop does not have the horsepower to do it as well as the lab.

2. I don't use Pro DPI, but most places won't do any "editing" for you, which this would be, unless you're willing to pay them a great deal more. Sharpen yourself, but don't do too much.

My work is done exclusively on metal, and usually at 20x30 or 40x60, so I have some experience trying to get the best image.

My advice would be to send some small crops of your existing files, sharpened by differing amounts (including none), and see how they look. I would not sharpen "stronger" than for regular prints. Most metal printers will send you a pack of 4 samples of the varying finishes they use. Although all four must be of the same image, I split up the crops, sharpening a repeated portion of each for later comparison within the same image.

[EDIT] - You wouldn't think that the other finishes would be useful, but they may give your pics a really cool vibe that you can't ever get with a print, so it's worth seeing what they look like anyway. The cost will be minimal, usually around $25.

Having said all that, my final preference has been to sharpen for the resolution I have, and let the lab upsample beyond that. I like the look of that much better than not sharpening at all. As long as you have a camera that's at least 12 Meg, you should have no difficulty whatsoever getting a really good-looking image at 20x30.