The crystal size is divided into three dimensions, Width (W), Heigth (H) and Length (L), which is common written as W x H x L mm³ . The careful design of crystal size is important because the price of crystal varies from crystal sizes. More important, the conversion efficiency has direct relation to crystal length. To select the optimum crystal height (H), the laser beam diameter upon the crystal should be taken into account. The optimum crystal height should be slightly (for instance, 1 mm to 2mm) larger than the laser beam diameter upon the crystal.

Both of laser beam diameter upon NLO crystal and tunable wavelength range have to be considered when designing the optimum crystal width (W). If it is a single NLO process, for example, frequency doubling 532nm, we select W = H. If it is wide wavelength tuning NLO process, for example, frequency doubling a Dye laser from 440 nm to 660 nm by using BBO crystal, the crystal should be tuned from q = 36^o to q = 66.6^o. The width (W) is set to H + 2 * tg[(66.6^o- 36^o)/2] * L. Because if the crystal height (H) is 4mm and length (L) is 7mm, the W should be approximately 8mm long.

Every NLO crystal has a standard length (L) for frequency doubling lasers with pulse width longer than nanosecond (ns). For example, the standard crystal lengths for BBO and KTP are 7mm and 5mm, respectively. However, OPO and OPA need longer length, for example, > 12mm for BBO, and the SHG and THG of ultrashort pulse lasers use thin crystals with length of less than 1mm.