Is there a way to find the length in mm between two chosen vertices while editing the mesh.
I don’t know, but a workaround in Edit Mode is to press N , then in the N Panel, scroll down until you see the Mesh Display tab.
In it enable “Edge Length”
Now select both vertices and press F to create an edge between them, the length/distance between both vertice will appear in the 3D View.
Then press X -> Only Edge/Face to delete that edge you just created while keeping the vertices where they are.
If both vertices are part of the same edge on your model already, no need to press F or delete the edge of course, the length/distance will appear when you enable the option in the N Panel
You can set unit measurement to meter in Scene data,scale to .001(it is under unit selection options None/metric/imperial) Then click edge length option in N panel, and select edge. Length should be in mm. .001 meter =1mm
If you want measurement in inches, select Imperial and scale setting of .0254.
When you use Scene unit measurement options, instead of measurements in Blender units, they will be in the units you specify in Scene Unit Measure setting. I am guessing you need the info for 3D printing a model.PM me for any help.
There’s also the Measure Panel Addon, included in trunk. Activate it in the N-panel, then select any two vertices and click “update selection”