What size envelope do you recommend for these cards?

Our OL247KW cardstock thank you cards are 4.25" x 5.5" once separated and folded, so as long as the envelope is that size or larger it should fit with no issues.

Do These needs to be Scored??

The OL247KW, Thank You Cards, are perforated horizontally across the center of the page and scored vertically down the center so they can be folded into 2 cards. You're welcome to visit the link below to request a few sheets for testing. https://www.onlinelabels.com/samples

Could I get your company to print on these for me? Or do I have to print myself?

Although we do offer custom printing on our adhesive labels, we don't currently offer custom printing on our White Cardstock (KW) material. https://www.onlinelabels.com/custom-labels

Can it be printed double sided?

The White Cardstock (KW) material is the same on the front and back, so both sides are printable. You would have to run the sheet through the printer twice if you don't have a printer that supports duplex printing. We do caution running the sheet through the printer multiple times or using the duplex printing option, as it could potentially weaken the perforations or cause jamming. If you do experience any feeding issues, try using your printer's manual feed tray with a heavy or thick material type setting. You are also welcome to visit the link below to request free samples of this product for testing with your printer. https://www.onlinelabels.com/samples