I use the Sony AVC render setting with Internet HD 16:9 preset and want to change the custom framesize. I am a bid unsure what the correct framesize would be here. Thanks.
Best thing you can do for Youtube at present is 1280x720 at 1:1 PAR.
A. Grandt's tests have amply demonstrated that 1080 uploads to Youtube are effectively reduced to 540 in processing. i just checked this again last week.