Tareq:

As someone who is just starting out, I would suggest that you start with smaller trays. They require less chemistry, and they are easier to set-up, take down, clean and store.

A great majority of my prints will fit into 8x10 trays, so that is what I suggest you start with.

You can always buy larger trays as well, but if you are like me, they will stay in the cupboard more often than not.

The largest size that is practical for me is 16" x 20", and I have a full set of those trays, but I don't use them often.

In the last couple of years I've had more of an inclination to print 11" x 14", so I put together a set of those trays, and use them a fair amount.

If you have the space, and you intend to use trays for washing your prints, it might be a good idea to go one size bigger than your usual print size for the print washing tray(s).