For Second Harmonic Generation (SHG) with CW lasers, a theoretical result from Boyd and Kleinman shows that optimum efficiency can be achieved when the ratio of the crystal length to the confocal parameter is 2.84, where the confocal parameter is twice the Rayleigh range. This is also true for Sum Frequency Generation (SFG) where the two pump beams should both be adjusted to have the same Rayleigh range.
Reference: Boyd, G. D., and D. A. Kleinman. “Parametric interaction of focused Gaussian light beams.” Journal of Applied Physics 39 (1968): 3597.
For Difference Frequency Generation (DFG) and Optical Parametric Oscillators (OPOs), optimum efficiency requires a confocal focussing condition where the Rayleigh range is half the length of the crystal. These focussing conditions also apply to pulsed lasers, but due to the high peak powers the spot size requirements are less sensitive. The user should be aware of the crystal damage threshold (see section 6 below) and not focus the beam too tightly as this may cause damage.
In general, a good rule of thumb is that the spot size should be chosen such that the Rayleigh range is half the length of the crystal. The spot size can then be reduced in small increments until the maximum efficiency is obtained.