Size doesn't matter...it's how you use it.

It depends on age more than anything. As you get older, you get
stronger. Since I don't know how old you are, I can't comment. Don't
use this as an excuse though, always try and go as big as you can.

^^^^^^^^^^^^^^^^^^^^^^^^
"I jump about 10 cms sideways then fall off that side "

Pull up harder on the handle so your feet stay firmly on the pedals
and/or pull the uni under you as you jump over (w/ your right leg if
going to the left). If you do the first and land ahead of the uni,
you'll have to do a correction hop to get the uni back under you.

Mostly it's just practice. Ride everywhere and hop up/down everything
of reasonable height along the way, esp stair sets.

The fastest sprinter on my high school track team was also the
shortest. He ran -SO SMOOTH.- If you could only see his head (like a
fence was obscuring his body), it would just glide by as he passed - no
bouncing up and down, like practically everyone else I've seen.

Unicycle For Christ
'MY VIDEOS-for enjoyment purposes'