This will fit a normal sized adult rat, and it’s pretty stretchy, so this should be a one size fits most situation.

Use whatever worsted weight scraps you have sitting around. Please, let me be the oddest one here- if you’re buying yarn especially for a sweater for a rat, you have more quirks than I do. Acrylic is probably best, because, again, handwashing your rat’s sweater? Don’t give yourself that much work. This should take you just a couple of hours to knit up.

For some reason, this pattern is taking off right now! I’m really enjoying all the pictures and comments. A few FAQs:
As you start decreasing, yes, reduce the width of the ribs to 1x1 or whatever makes sense.
Probably not a great idea for the unsupervised rodent. Rats can and will quickly get themselves out of it and chew it up. I would hate to hear of a rattie that got tangled in sweater shreds or ate yarn and hurt itself! Think of this as the dysfunctional cousin of those baby photo shoot props- stupid cute, but not for actual living in.

Feel free to send the pattern on. Just keep my name on it!