Knots tend to be about 0.4mm to 0.5mm each. (My knots tend to be more like 0.4mm.)
The amount of length knotting would add depends on how many pearls are in the strand.
If the pearls are small and numerous, the knots will also be numerous, so knotting will add more length than if the pearls are larger and fewer.
Also, when stringing with doubled thread, I cut a length of thread 5 times the finished length of the strand. You don't want to run out of thread, which is cheap-- it's a nuisance to start over.