Set up your title block in paperspace at a scale of 1:1 ie actual size then open a mview window at the scale you want the final plot to be. Then plot the paperspace titleblock at 1:1
We use an A1 title block sheet 841x594mm. To plot A3 1/2 size we use a plot scale of 1:2 for the whole titlebock the window is still then at a correct scale but 1/2 the A1 scale.
A1 at say 1:250 now 1:500 at A3