What is the approved size of the P trap in the kitchen sink?

1.50" for kitchen

1.25" for bathroom

Good plumbing practices one would use 11/2 in for basins and 2" for kitchen sinks would be ideal to allow for routine snaking and better scouring action.

Using barely minimum code standards is not what one should use in their own home or any building you may have to service one day

Many times a kitchen sink will have a garbage disposal or dishwashing machine or both attached.

All these devices will contribute to the piping walls closing up from grease and soap scum and food particles caught on the waste that adheres to the piping.

Also running a snake or water jetter through an 11/2 line is challenging at best.