Documenter doesn’t have a nice built-in way to do it. Hashing out an interface for it and implementing it is an open issue: Controlling output text width in doctests · Issue #942 · JuliaDocs/Documenter.jl · GitHub
However, as is mentioned in that issue, it might be possible to make the outputs consistent by setting ENV["COLUMNS"] and ENV["LINES"] in make.jl (see also ? displaysize). And you can use the doctest = :fix feature to update your doctests when you change the number of lines and columns you want printed.