The length of cavity is composed of crystal part and air part. The air part is the part between crystal and in-coupling mirror. In the first attached figure, we can see the lateral side of OPO housing. The red line stands for the cavity length. The way to estimate it is to seperate it to three parts as it is shown in the first attached figure. 

The blue line is shown in the second attached figure. It is roughly 14mm.

The green line is shown in the third attached figure. Since we can see in the attached  fourth figure, Matteo put the crystal roughly in the center of POM_bridge. So the second part should be half of bridge thickness. It is roughly 25/2 = 12.5mm.

The last purple line is half the length of crystal. We can know from the attached figure 5, crystal length is 9.3mm. So this purple line length is 9.3/2 = 4.65mm.

So the total length is 14+12.5+4.65 = 31.15mm. It concludes 9.3mm of KTP and 21.85mm of air. The real length should be 9.3*n(KTP)+21.85*n(air). The refraction index of KTP for infrared is 1.7379. So the real optical length is 9.3*1.7379+21.85 = 16.16+21.85 =38mm