So you're at the anchors. You clip your rope into the quickdraws and yell "take." The rope is going through your tie in points. The knot is two inches long from your tie-ins to the draws. The draws are eight inches long. So your belay loop, with your 3-inch carabiner is ten inches away from the anchor, and even if you get that in, which you can't, because your belayer has taken your slack in, so when you stand up, you get pulled down -so now you're six inches away from the anchor... One anchor, because the two anchor bolts are also several inches away from each other. You yell down for slack to clip yourself into the first anchor -but you can't clip yourself into the second anchor, your belayer can't feel your weight, you untie, clean, and rethread, now you ask to take, but the knot is so big it's just pulling on the rap ring and you can't feel it because you're direct with your biner -which is holding all your weight now, and you can't get off of it because you're trying to pull yourself up by the thing you're hanging from.

You want the slings to be longer than the draws so you have slack to play with. You want to know when your belayer is taking you in, or giving you slack.

Most people use 60cm slings, I use 80cm, and once had some 100cm slings that I retired -I wish I still had those 100s, they were sweet.

My suggestion is, don't buy anything, ask people what they have, and ask to borrow it. Chances are, if you go with 3 different climbers, they have 3 ever-so-slightly different systems, and you should try them all. Some people use short slings, some use long ones that are knotted, some use PAS, some use quickdraws, etc.