There is a standard Scaling system for CAD for text sizes.
Its based off the Scale of the drawings and Text in Model Space (where it should always be, unless its Titleblock information---start the debate
).
1" = 1" which is a viewport scale of 1/1xp = 0.125" text size
3" = 1'-0" which is a viewport scale of 1/4xp = 0.5" text size
1-1/2" = 1'-0" which is a viewport scale of 1/8xp = 2.25" text size
1" = 1'-0" which is a viewport scale of 1/12xp = 1.5" text size
3/4" = 1'-0" which is a viewport scale of 1/16xp = 2" text size
1/2" = 1'-0" which is a viewport scale of 1/24xp = 3" text size
3/8" = 1'-0" which is a viewport scale of 1/32xp = 4" text size
1/4" = 1'-0" which is a viewport scale of 1/48xp = 6" text size
3/16" = 1'-0" which is a viewport scale of 1/64xp = 8" text size
1/8" = 1'-0" which is a viewport scale of 1/96xp = 1'-0" text size
3/32" = 1'-0" which is a viewport scale of 1/128xp = 1'-4" text size
1/16" = 1'-0" which is a viewport scale of 1/192xp = 2'-0" text size
1/32" = 1'-0" which is a viewport scale of 1/384xp = 4'-0" text size
If your doing metric drawings just take the text size an multiply by 25.4 and thats your text size.
1:200 scale is eq. to 1/16" scale so the text size would be 24"X25.4 = 609.6mm
1:100 scale is eq. to 1/8" scale so the text size would be 12"X25.4 = 304.8mm
1:50 scale is eq. to 1/4" scale so the text size would be 6"X25.4 = 152.4mm
If your doing text in Paper space (layout space) then you can just use the same scaling for the text sizes but scale the text down by the viewport scale (ex. 1/96 , 1/192 , 1/128...)