This tumblr blog has many great resources and answers: https://howtobeaconartist.tumblr.com/resources
I myself have only sold online and for me, people like to buy mini prints (6x4 or 7x5 inch) more than big prints, probably due to the cost of shipping. I've heard this is different than physical artist alleys where people are more likely to buy bigger prints (8.5x11, 11x17, etc.) but that may also change if, for example, you sell mostly mainstream fanart vs. niche original art.
Be sure to check each printer's specifications because some will require you to use different bleed sizes, file types, etc. Most will have a template you can download on their websites.
Companies I've tried that offer low minimum-quantity-to-order:
Cat Print was great for larger prints but I was very disappointed with the quality of their smaller cardboard prints (6x4 inch or 7x5 inch).
Vograce is another popular manufacturer that offers keychains, prints, stickers, standees, washi tapes, charms, etc. They're located in China so you'll have to account for international shipping fees and time. I only made PVC keychains with them but my friends who've used their prints/sticker/acrylic charm services have been happy so far. BUT sometimes they are very slow to respond or manufacture, especially during convention season.
Stickerapp does stickers only and I think their gloss option is nice -- the matte wasn't to my liking.
Zap Creatives is based in the UK and offers keychains, stickers, pins, etc. I printed an acrylic standee with them once and the quality was great.
Mixam does books and prints. I was happy with the quality of my prints but a friend was told by a Mixam rep to increase their order count to meet their - I guess - invisible MOQ so that's something to be wary of. Books didn't have the requirement. Shipping even within the US can be expensive.