Never too late (well... :-) - but wait til April at least before pruning hard down to a few branches... but do more research then before starting out. You're still growing 'sticks' and the lower trunk needs to be fattened up before being a 'bonsai', which means actually leaving one branch to grow uncut below the lowest one you actually plan to use as your 'first' bottom branch... that wild one, to be cut off in years to come as your design develops, will help fatten the lower trunk over time and then become a 'sacrifice' branch.

Will I be able to reduce the size of the leaves and flowers very much, and how? How much of the root mass can I take off in the first pruning? How long will it take to develope a good bonsai shape?

The flowers won't reduce... most of these bonsai allow only one or two flowers once established (you'd pinch the buds off), but of course you can work on the leaves like any other bonsai. Take off 1/3-1/2 of the ball depending on the pot you want to use and how the roots look. And how long it takes to become a bonsai depends on too many factors to think about predicting now.

Ok, thank you all for helping me :)