Good question.
You’ll want to have licks and rolls that are either 1 measure in length or two measures in length to start.
Once you master those you can create longer combinations.
So if you need three measures you could combine a one measure lick/roll with a two measure lick/roll.
Or play three one measure lick/rolls to create three measures.
Same with four measures. You could play the two measure lick/roll twice.
It all depends on what licks/rolls you already know. At first you’ll be repeating a lot of licks/rolls because your vocabulary will be smaller.
I think of the measures as blocks you can build together to create different amounts of musical time.
If you still have questions let me know.
– Mike