In our office we usually use a 2.0 mm text size for Dimensions, Zone Stamps, Labels, etc. in Construction Documents.
It also depends on how much space we have in the rooms, sometime we can go up to 2.5 mm. But there were times when we had to go down to 1.8 or even 1.5 mm because the rooms were so small and we had to cram like 4 rows of text into a Zone Stamp.
But below 2.0 mm it gets hard to read when it is printed and photocopied.
For things that have more space like notes, schedules and stuff, it can be more: 2.5-3 mm, which is usually very readable.
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