Everything gets a T-shirt size. Roughly, "small" is no more than a couple person-days, "medium" is no more than a couple person-weeks, "large" is no more than a couple person-months.
Anything beyond that, assume the schedule could be unbounded. Figure out how to carve those into a series of no-larger-than-large projects that have independent value. If they form a series of iterations, don't make any assumptions about whether you'll ever even get around to anything but the first one or two. That just compromises your ability to treat them as independent projects, and that creates risk that you find yourself having to worry about sunk costs and writing down effort already expended when it eventually (and inevitably) turns out that you need to be shifting your attention in order to address some unforeseen business development.
At the start of every quarter, the team would commit to what it would get done during that quarter. There were some guidelines on how many small, medium or large projects they can take on, but the overriding principle was that you should under-promise and over-deliver. Lots of slack (1/3 - 1/2) was left in everyone's schedule, in order to ensure ample time for all the small urgent things that inevitably pop up.
There was also a log of technical debt items. If the team finished all their commitments before the end of the quarter, their reward was time to knock things off that list. Best reward ever, IMO.