Take a look at
Universal Orlando Crowd Calendar
Touring Plans shows a couple of weeks worth of info for free. They have subscriptions to their crowd calendars for both WDW and Universal. While I have no financial interest in it, I have been using Touring Plans for several years and (before we moved to TV) used it to plan our trips - with a subscription, you can see 365 days out. So far, I have found them to be wrong once. If you look at their Lines computer/phone app. you can see surprisingly accurate wait times for each attraction.
As big a factor as crowd size is when you get to the park. Simply put, be there by 8:15 to buy tickets and be in line for rope drop (which often happens 15-30 minutes before the advertised time. I'd suggest doing Islands of Adventure first and going *directly* to Harry Potter. Do it that way and you can probably get in two rides before the lines get totally crazy.
Good luck!