Dimensioning: manual, and automatic

External dimensions of a building can be done manually, or automatically. Here's how.

Move Grid bubbles away from the building

To make room for the external dimensioning of the building, make sure the grid bubbles are pushed back, say, about five bubbles'-width away from the building.
To do this, go to the Grid layer [1], and make sure to Shift-move them, so as not to move them out of alignment.
Do this at the top for the vertical gridlines set [2], then click the handle at the tail end [3] to move the bubbles at the other end.
Repeat the process for the left and right bubbles.

Dimension Exterior Walls – the manual method.

For this, used your Saved View of the Main Floor Plan to go to the Main Floor Plan on the Main Floor design layer, and use the Constrained Dimension tool [1], in Chain-dimension mode [2], then click at the outside corner of the building [3],
then at the Midpoint of the first window or door opening [4]
then pull away to indicate the offset that you wish for the entire chain [5].
NOTE: Make sure this is clear of the entire building, including projecting bits further along the wall, but leaving room between it and the grid bubbles.
After that, you need only to click consecutively at each of the relevant points: the Midpoint of the next window or door [6], the centreline of the interior wall meeting the exterior wall in a T-junction [7], or the outside corner of the building [8].
To finish the chain dimension, remember to double-click at the last point.
[With thanks to Kim Abbott/ID2B for the base drawing]

Change dimension font size to 9 or 10pt.

Dimension text should usually be 9 or 10pt in size (anything less risks being too small to read; anything more is wasteful of space). While the dimension chain is selected, change its font size to either of those sizes, through the Text menu.

Repeat the process within the Annotation part of the viewport.

To demonstrate why we should carry out all our dimensioning – and other annotation – strictly within the Annotation part of the viewport, now go to the Viewport of the Main Floor plan on the sheet where it is shown by itself, at a scale of 1:50, and repeat the process, positioning the new chain slightly above the previous one.

Now duplicate the viewport, change its scale to 1:100, and position it next to the original 1:50 viewport.

Change your Zoom to Normal View – i.e., the size simulating the size of objects were the drawing to be printed.
Note how to the dimension text that we created in the Annotation part of the Viewport has remained at the stipulated 9 or 10pt [1] – while the text of the dimensions we created on the Design Layer has shrunk to half its original size [2], following the change in display scale, and is now quite unreadable.
This is not the only reason why we should always dimension or annotate within the Annotations part of viewports, and not on the Design Layer – but it's probably the main reason.

Dimension Exterior Walls – the automated method.

Now that you've seen how exterior dimensioning is done manually, you might want to avail yourself of the automated method.
Go to the Main Floor Plan (either directly to the Design Layer, or by double-clicking its viewport on one of the sheets, and going to that design layer that way)
then choose AEC > Dimension Exterior Walls…
You get a dialog offering you this operation on any of the design layers applicable. Choose the one you want (say, Main Floor, for now), and note the other default settings, but don't change them. Then OK…

Result.

Within seconds, it will have completed a exterior dimensioning of the entire building at that level, in three separate chains: for openings; for interior-wall T-junctions; and for overall length of each exterior wall.
Because dimensions should not remain on a Design Layer, we now remove this dimension set from here, to paste into the annotations part of an appropriate viewport: while they are selected, immediately Group them (⌘G ; Windows: Ctrl+G), then Cut them to memory.
NOTE: Make sure the Group itself is in the None class, and not some obscure one that might be turned off in the destination viewport.

Paste into the Annotations part of the relevant Viewport

With the dimension group still on the Clipboard, go to the Sheet where this plan is shown in a viewport set to 1:50 (not 1:100 – too small), double-click that viewport, and enter its Annotation section. Then paste in the dimension group anywhere.
NOTE: Paste-in-Place won't work properly, because the coordinates at the Design Layer and within the Viewport are different.
The trick now is to match the dimension set with one of the building corners: move it approximately into position, then zoom in on that area.

Drag to the relevant corner

If you hover with your cursor over the point where the two corner witness lines would meet, a screen hint Point will appear: click and drag it to the corner in question, and you're done.

Make manual corrections, where necessary

The automatic dimensioning is not always faultless: things like window label bubbles can throw it off – so make corrections manually where needed.
NOTE: Consider not using window and door label bubbles in this case.