Default value of W/L of nmos in LTSpice IV
If you read the help file, all is revealed.

First, read the section on the M circuit element. For the level 1 through 3 MOSFET models, the default L and W values are given by the parameters defl and defw, respectively.

These parameters are defined in a .OPTIONS card. The help file page for .OPTIONS tells you the defaults for defl and defw are both 100 μm. 

So if you do nothing, your MOSFETS will be assumed to be 100 x 100 μm. 

However, if high level parameters VTO, KP, LAMBDA, PHI, and GAMMA, (particularly KP if I recall correctly) are specified, the low-level physical structure parameters are ignored, so these default dimensions won't affect the simulation.