I use the default parameters for SURF, except the hessian/response threshold that I change accordingly to the image size. See Sections III-A and IV paragraph 2 of this paper. The hessian threshold should be low enough to have around 400 features extracted from the images and high enough to avoid bad features or too many features extracted (with images at high resolution). The 400 maximum features parameter was fixed experimentally, mostly with the challenging UdeS dataset. This parameter can be lowered to increase the WM size manageable in real-time.