I think that's the right thing to do. A further idea would be limit the brick sizes to what would fit, sort of snap the value to the closest precalculated value that fits. That might be more user-friendly.
Your last set of pictures looks very realistic! Although I don't like the black mortar in the bottom two. The top-left mortar color is the best. Obviously you are letting users set the mortar and brick colors but I just wanted to comment on them anyway.
Also in the last set of pictures, the shadow/light on the brick edges seem to have gone. The bricks look very flat now. What happened there? Did you set the mortar depth very low?
Your last set of pictures looks very realistic! Although I don't like the black mortar in the bottom two. The top-left mortar color is the best. Obviously you are letting users set the mortar and brick colors but I just wanted to comment on them anyway.
Also in the last set of pictures, the shadow/light on the brick edges seem to have gone. The bricks look very flat now. What happened there? Did you set the mortar depth very low?