5x5 and 6x6 (Those are the ones I have) I would recommend 7x7, but I don't have one. After 7x7, it doesn't really matter because bigger cubes tend to be more expensive and there aren't events for them in the WCA. However, if you have bigger cubes and a YouTube channel and do full solves of them or something, people are usually going to watch what you did more. In the end it's up to you.
I'm working on improving my unofficial Kinch Score the last few days. Now that I actually know what it is, I've improved from a 6.22 Kinch to an 8.68. Hope I do well at my first comp in about 3 weeks!
