Do unequal step sizes in staircase procedure affect the convergence?
It is going to mean bad things. If you can only create stimuli with a predefined non uniformly sampled set of "levels" then the the corresponding percent correct that the staircase will track will depend on the threshold of an individual observer. For example, if you can create stimuli with levels of 0, 1, 2, 3, ..., 100, 110, 130, 170, 250, 410, where for small levels the up and down step sizes are equal, but for large levels the up step size is twice the down step size. This means the percent correct point is going to be different depending on the threshold of the subject. You wouldn't have to worry about it if you use something like the method of constant stimuli.