The following can be used for rough estimations ..
First of all, have a look (table below) at basic simplified equation for a buck converter ..
Select MOSFET which is rated for at least 2 times Ipk (Iload max in your case is 2A, and Iload min has to be specified by yourself, say, 20mA - no external load connected) .. so any ≥ 4A MOSFET should do ..
VDS voltage should be at least 2 x Vin max ..
P{ower} rating of the MOSFET is strongly related to its Rds(on).
To estimate it use the following formula:
P(mosfet)=[Ids(1-δ)²]*Rds(on) .. δ-->duty ratio, for estimations you can use δ ≈ 50%, Ids ≈ Ipk and Rds(on) from MOSFET's Data SHeet ..
Regards,
IanP