Yes, mine were standard sizes. I actually had a couple that weren't, but I made the hole larger or smaller, depending on the case. So, I also ended up doing some drywall and siding along with installing my windows. I blocked off about half of the old windows as well.
Windows and siding. Two of the easiest jobs with some of the highest markups. That's why so many people are in one business, the other, or both. Are you going to do it, or have someone else?