It seems so easy to make it available. I tried digging around the SDK to see if it was possible through that route, but that stuff is beyond my abilities.
The idea of drawing unmodified solids on a hidden layer seems like just as much work as keying in the dimensions for custom properties. We really shouldn't have to do things like that. I emailed TC, but they haven't responded and I don't think they will.
If I understand this thread correctly, Alterations to the tools has been asked for previously including a proper BOM, here is a 'long' thread on a similar subject back in 2011, http://forums.turbocad.com/index.php/topic,6949.msg44377.html#msg44377
One problem is that 3D ACIS 'size' isn't included in standard object properties even through the SDK, so as far as I can tell one would not be able to just enter a simple command like 'x size', to do it via the SDK one could use the bounding box method, but even then is not a simple task to write a program for all circumstances and is further complicated if the x/y/z isn't the natural x/y/z (like when using the rotated box tool).
A very simple example macro of getting the size of ACIS solids drawn on the World workplane is - draw 3 boxes on the world workplane, explode once so they are all ACIS solids, give then each a name, paste the code into the macro palette, select the three objects then run the macro.
OK its a bit of a rough way of doing it, just posting as a very simple example
set sel = ActiveDrawing.Selection
Numsel = sel.count
If Numsel > 0 then
For i = 0 to Numsel-1
set gr = sel.item(i)
nme = gr.Name
set bbs = gr.CalcBoundingBox
set bxmax = bbs.max
set bxmin = bbs.Min
L = bxmax.x - bxmin.x
W = bxmax.y - bxmin.y
H = bxmax.z - bxmin.z
MessageBox ("Object Name = " & nme & vbnewline & vbnewline & "Length = " & L & " Width = " & W & " Height = " & H)
If Numsel = 0 then MessageBox("Nothing Selected")
Set gr = Nothing
Set sel = nothing
Set bbs = nothing
Set bxmin = nothing
set bxmax = nothing