My lemon tree seems healthy - plenty of flowers and new leaves. When the fruit is 1cm long, they stop growing.
Ensure you're not over or under watering - push your finger or a cane right down into the soil to test soil moisture, prior to each watering.
Fertilise lightly once every season with a specialist citrus food, to ensure your tree has all the required nutrients available to develop fruit:
Keep in mind that young trees will put on more flowers/fruit than they can sustain, so these are best removed in the first few years:
Answered: 21 Oct, 2020