Recently I observed that the leaf size of my mint plant has started to shrink. Previously the leaf size used to be around 2-2.5cm. Presently it has shrinked down to 1-1.5cm. What might be the reason? How can I rectify this issue. I water the plants daily. It recieves 4-5 hrs of direct sunlight. I tried to provide some shade however, the leaves size is still not growing. I feed them with urea once every month.
This is the same effect we see when a clump of mint is planted in open soil - it starts to form a circle growing outwards, and as the roots get thicker in the middle of the circle the leaves get smaller but those on the outside remain big and healthy since the roots have room to expand. We can maintain larger leaves artificially by heavy feeding but even this starts to fail.
The remedy is to turn the plant out of the pot, extract some healthy roots and plant in fresh soil. In your case you might be able to dig out one half of the mass of roots in the soil and use this for fresh propagating material, leaving the other half still producing leaves. Then you have the basis for a renewable system, one young and one older planter, with each being renewed at a different time to ensure a continuous supply of leaves.
By the look of it, I think Colin is right about it being rootbound. Plants seem to get smaller leaves, sometimes, when they can't grow to their full potential.
Low phosphorus is also known to reduce leaf size.
