What size is this? It does not give an option to choose a size for check out.

There is a size option if you scroll down the main page a little bit. Before adding to cart

Do you sell 3-6m cotton pajamas?

Yes, we have 3-6 months organic cotton and regular cotton footies. Please reach out if we can help you with a specific print or color.

Does the 3-6 months onesie have hand mittens/cuffs?

It may depend on the print and style vs the sizing. I bought 2 which both had them but the fabric was different - one seemed like summer and one for winter. I had a yellow 0-3 and then a 6-9 month one.

I don't see 0-3 month size option. Is the "newborn" size (as opposed to NB 0-8)suitable for babies 9-12 pounds?

0-3 months is larger then Newborn. Some of our 0-3 month inventory is temporarily out of stock. Please view ASIN B08F3HZR89 size 0-3 months as an alternative option. If this pattern is not what you are looking for, we have our magnetic footies in a soft modal fabric available in 0-3 months. Please view ASIN's B08FVRSNYF or B0831VF77S for patterns available in 0-3 months.