I am new to offering my photos for sale and using SmugMug. My flow is with mostly Lightroom and cropping at 8.5 x 11. Now, for the question... what is the best size, pixels/inches, to upload to be able to use/sell most products available? Oh yes, I have a Pro account. Thanks!
Don't crop or resize at all. Upload all of the original pixels, this way you have the filesize to be able to get the largest prints available, and the largest variety of sizes (not limiting to 8.5x11). You set your galleries to Proof Delay, so that if you need to make an adjustment (like cropping, for example) to the order before it goes to the lab, you can.
If uploading bandwidth is an issue, and you have lots and lots to upload, well then an approach many take is to resize their images to be just above the pixel dimensions required for the products they want to sell, and then also compress them in LR, maybe using LR7 or something like that when you export. Upon order, you will of course replace the file using Proof Delay, and replace with the full-size file, and perhaps only compressed using LR9.