All the things mentioned before were good suggestions. I just want to mention my opinion about big cubes, as someone who kinda went down that rabbit hole (I have up to 9x9).
If you're not sure, don't get 6 and 7 unless you feel like you have enough money for it to be worth it (since they're more expensive, you could get other cubes for the same price.) But I think if you keep taking cubing seriously, you'll likely end up getting 6 and probably 7 at some point too. If they really aren't your thing, don't bother getting 8, 9, etc. like I did. See picture of where my 8x8 and 9x9 have been for several months:
View attachment 8972
Yeah, hiding in the dark behind some random 5x5s