Im wondering what size should I choose 5mm or 7mm does it really matters?
Are Xmas tree spikes all that superior to pyramid in sprints?

You could try either spike length and see which ones work the best. I prefer a longer spike for sprints. If you are a mid distance to distance athlete, then the 5mm would work fine. As for the xmas trees, they were designed to be safer for track surfaces. I have heard of athletes mixing their spikes and putting half pyramids and half xmas trees on their spikes. Hope this helps.

xmas tree and pyramids have their own feel and advantages. from a pure physics standpoint, the xmas trees are superior because they return some (note "some" meaning not all) energy back to you by compressing the track underneath. bear in mind though, that if the track is not rubberized or is dirt the xmas trees will have no advantage over pyramids

people have success with either pyramids or xmas trees. its purely up to the person. try a set of both and see what you like best.