Harvest:
Harvest spring planted kohlrabi when the enlarged stem is 1 to 2 inches in diameter. Harvest summer and autumn planted kohlrabi when the stem is 3 to 4 inches in diameter. Harvest giant kohlrabi varieties when they are 8 to 10 inches in diameter.

For more information on growing Kohlrabi, check out this great guide from Harvest To Table:
https://harvesttotable.com/harvest-store-kohlrabi/