1. The size of a file affects the speed that takes to make the render preview in ae?
2. I used to edit my projects in ae and sony vegas. I render with quick time in ae and than wmv in sony. I put myself a question what is the best format to render in ae and the upload on youtube. Best result came from flv... I want to know your opinions...
3.What software to use to compress files? adoe premier / sony vegas / final cut pro are there any differences?
This is a matter with a lot of variability due to time and computing restraints/advantages. Also, opinion. What works best for you is really the right answer. However, learning other people's workflow is good for comparison. Here's what I did:
Render out any element from AE in QT (best quality/huge files). I put the video together in Premiere (add sound/music, and basic scene transitions). I prefer to render from there to 1280x720 mpeg II (faster upload to web) but you do lose quality. The network I worked on has upload limits, and if I uploaded a huge QT file, everyone at work phoned me that they couldn't use the internet (example of circumstances). Youtube has good conversion software and as long as you give them a high quality version, it looks good. If you are doing work for a client, obviously, provide them with the highest quality render you can.