There is no limitation on voxel size but 1mm is generally optimal for T1 brain scans. In our experience, BrainSuite generally works with tumor brains in the sense that the labeling of regions away from the tumor region is relatively unaffected. The execution time can vary for the tumor cases though it should not be too high. Did you get a chance to try normal brains with BrainSuite to make sure that the long execution time on your computer is not related to tumors?
If possible, it would also be helpful if you can post a screenshot of your results with the tumor.