This may help as a starting point
https://www.talkphotography.co.uk/tutorials/which-is-the-right-softbox-for-you.154/
But more useful, specific advice will depend on what you know and we don't . . .
Full length, half length, head only?
Glamour portraits, character portraits, something else?
Small room, large room, dedicated studio?
All that we do know, right now, is that you're using a hotshoe flashgun. In theory, using a s-fit adapter, you can use any size of softbox but in reality the fixed reflector will create uneven lighting with a large softbox, some people think that up to about 90cm is more or less OK, but I think that 60cm is about the upper limit.
Large softboxes don't really work in small spaces, and they certainly don't work with low ceilings, unless your sitter is seated.
If you're photographing a teenager or child with perfect skin and perfect bone structure then you're probably better off without a softbox, which tends to create soft light and gentle shadows. If you're photographing their mother then a medium-sized softbox, used pretty close, is often about right. If you're photographing their grandmother, the bigger the better.
One bit of advice that really does matter - don't get one of the cheapies that don't have a recessed front diffuser, waste of money.