Is it possible to select all lines in an illustrator document of which the weight is below a certain point size? I'm working on an architecture publication and have to rescale all the plans. And resizing means that some line ticknesses become unprintable.
Yes, You can.
There is a script that does the job.
The script is not mine and I'm just sending the link for that script.
Also, I'm sending an image showing the script in action.
This is an alternative method that might help you.
You could change your transform settings, so that when you scale, you don't scale the strokes. That is assuming the lines have strokes, and are not expanded to shapes. To do that click Edit > Transform > Scale, and deselect the option "Scale Strokes and Effects", hit OK. Now when you do a scale transform the strokes will not be scaled.
The Scale transform setting
The result of scaling with that option unchecked