After having collected all the feedback, let me summarize the findings. Uniformity can be evaluated at different scales, either across the sample area or for high resolution homogeneity.
As to homogeneity across the sample area, this is achieved by having a precision syringe pump delivering the matrix and by selecting a line distance of 5 mm or below.
The drop size depends on the matrix solution, matrix flow, gas pressure, distance of capillary end to gas tube and distance between nozzle and sample. We found good starting parameters to be 1.2 bar gas pressure and extension of the glas capillary out of the stainless steel capillary of 0.5 to 1 mm. These two parameters are the most critical.
In case where the matrix deposition is too wet, you might want to decrease the line distance. This will result in the same amount of matrix being delivered to the sample, but at a lower rate. Alternatively, or in combination, you can increase the gas pressure to deliver a dryer spray.