Hello, 

Thanks for reaching out! Here is a helpful UMN Extension article about growing kohlrabi: https://extension.umn.edu/vegetables/growing-kohlrabi

It is recommended to harvest the kohlrabi when the bulb is 2-3 inches wide, and the stem diameter is approximately 3 inches wide. You could always consult your seed packet (if you still have it around) to see when the expected harvest date is. It's better to harvest when the kohlrabi are small -- if they become too large or over-mature, they may turn tough, woody, or fibrous. 

Good luck!