Size-wise, the Dual GeForce RTX 3060 offers a 2-slot design and is only 200mm long, so it’ll fit easily into a wide range of chassis options without blocking access to other expansion slots.
For those just starting out, I also like the RTX 3060 here, which is a great pick for those on a budget, since it comes with 12GB of fast GDDR6 memory that allows it to run pretty much any GPU ...