I think the ability to use some kind of arrow tool to quickly count how many squares an objet is would be a nice feature to have when building a dungeon to exact dimensions. That or the ability to resize an object to say 2x12 to make that stretch of wall without having to count every little grid square.
Or even some kind of visual que option in the grid squares themselves might be another solution. Say every 5 squares the lines are a different color to make counting a bit easier.
Of all the options the resize an object to exact number of pixels/grid squares is probably the best but if not possible any of the options listed would be an improvement.