Low Power Switching application With N-Mosfet
Seems like you need a number for what is an acceptable loss rather than "..kept to a minimum", which will usually lead you down the wrong path. 

For example, if the voltage is 5V and the power is 10W the current will be 2A. A maximum acceptable loss might be 2% or 100mV, meaning 200mW loss in the MOSFET when 'on'. 

That implies an Rds(on) of 50m\$\Omega\$ or less. You can search for MOSFETs with such a characteristic. My go-to SOT-23 N-channel MOSFET for switching significant current is the AO3400 which has a guaranteed Rds(on) of 33m\$\Omega\$ at 4.5V Vgs (it will be higher for higher Tj, but still less than 50m\$\Omega\$ under reasonable conditions). It's a very cheap small part. 

You can find other parts in SOT-23 with even lower Rds(on) or go to other packages (NXP - now Nexperia has some nice parts) that have a bigger die and are better (for a price). For example (at the low end of Rds(on), the PSMN0R9-25YLC
 which has 1.25m\$\Omega\$ maximum Rds(on) at room temperature. 

It's not a good idea to go completely overboard on the Rds(on) parameter- the larger MOSFETs tend to have much higher capacitance, so they switch slower, and of course they tend to cost considerably more. That particular one is USD 0.54 in 1,500 quantity vs. USD 0.085 in 3K (one reel in each case), so more than 6x the cost. Actual cost differential in large production quantity is likely a larger percentage. Somewhere in between might be a better choice. 

tl;dr  Determine a specification for performance and other characteristics and do a parametric search. Iterate if necessary.