If you're using a full-size ATX case (which is what you have in your build), you can fit either an ATX or a mATX motherboard into it. mATX tend to be a bit cheaper and have fewer options for extra storage drives or graphics cards, but since you're sticking to a single storage drive and a single GPU, you have way, way more space than needed even for an mATX. However, if you're going to have a full-size case anyway, you might as well take advantage of the space and get a bit larger of a motherboard so you'll never have to worry about expandability down the road. With that said, the cheapest available right now is probably the
ASRock B450 Pro4 for $70 (after a $10 mail-in rebate), and it will be perfectly fine for your use case - you could even overclock your CPU a bit and this board will be fine. If you find that you'd rather save a few bucks, grab an mATX board like the ASRock B450M-HDV R4.0.
As a note, the stock cooler with the Ryzen 2600 is very good for being a stock cooler, so you can use that without any need for something beefier.