The pressure available to you is very important, as others have pointed out, and you have no control over that but buildersmate's advice is precisely correct. Since the length you require is in effect dictated to you, the only factor you have control over is the diameter of the hose. Yes, a larger diameter would reduce the resistance to flow through the hose and the more rigid and smooth on the inside it is then the less resistance you will suffer. Fifty metres of hose is quite a substatial length and in actual fact, if you were to conduct a test, there would come a point whereby continually lengthening the run of hose would result in a mere trickle even on the horizontal with the tap fully open - but currently a 30cm rise probably does not have the biggest effect although (in your test) you could cut the flow completely for a good while simply by raising the end of the hose 30cm. My advice would be to get a larger diameter hose and try to make sure it is unlikely to kink/collapse (thicker walled is better) and is the smoothest one on the inside on offer. Then look after your hose, coiling it carefully for storage and fastidiously avoid kinking or squashing in any way.