how to size FET for 2A for DC DC converter?

[chinito]

I'd like to know how do you size say a PFET for 2A of current in a DC DC buck converter?

 

[IanP]

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