I don't think you need a bigger kite IMO...
8m in 15-20 knots is big enough at 60kg, even when learning. Maybe it would be easier with a 10m...well
But it sure is easier when you're (over)powered. You don't need to work as much and you're lack of technique won't really shows...but it won't help you in the long run...
It's all about technique. Do more waterstarts. How you edge and position your board is really important. It's normal to dive the kite hard when waterstarting. Don't sheet the bar too much, it will choke the kite. You'll see...you'll be up and kiting in no time!

I learned with my 8 m in 15-25 knots and I never felt I needed a bigger kite...only a smaller one for >25 knots.