The smaller digit is your better option, cannot remember but all you do is divide it out and look at each compared. The last clamp I had was 5/16 size, metal not plastic digits. They will be no good if not cleaned properly every time and dried well before putting up though.
I use a pen, the KBTAT to be specific, less than 40.00 for brand new set. There are sets others have slightly used for less, on facebook there's usually some one with one or two (even the clamp style) for much less than what you'd pay new. As long as you clean them, perfectly usable. You may need to order a new needle, but that's relatively inexpensive compared to the rest of it. Ink needs to be changed bare min every 6 months, India ink from a hobby/craft store works well. Dark ink is your best; black, blue, or green but there's nothing that says you can't use some other "off" color if you choose. The tattoo just needs to be easily seen. I also use orajel or if I can afford buy Gigi's numbing spray or a similar one for waxing/no-no's hair removal....works awesome. I have used my pen tat on every thing I've had from itty bitty NDs to EL ears
I write in block style and haven't had a complaint on the table. I have had a few that were tattoo'd really young that faded with age (my ink was over the 6 month mark and I was just trying to get them done for showing).