How can the forum possibly answer that, since it depends on how good your eyes are ?
Work it out for yourself, you have the current display working so measure at what distance you can read the display, then do the summs.
Lets say you can read the level on the current display at 2M distance. To then be able to read it at 100M your going to need a diplay that is 50 times bigger, say 1.2M high, good luch with that.