GPM into a hyd motor determines the speed.
How fast do you want the trimmer to run?
Are you thinking direct drive or belt and pulley?
Are you using .155 in line?
10 GPM, 1.0 cuin hyd motor = 2,310 rpm
5------, 1.0 cuin ------------ = 1,155 rpm
10 ------.5 cuin ----------- = 4,620 rpm
5-------.5 cuin ----------- =2,310 rpm