Dimensions
For anything sold by size, the question behind the purchase is always the same: will it fit. Dimensions answer it on the model itself, with the measurement drawn where the customer is already looking.
They update as the configuration changes, so a customer dragging a width slider watches the number follow.
The six types
Linear measures a straight distance between two points. Width, height, depth, the gap between shelves. This is the one you will use most, and for many products it is the only one needed.
Angular measures the angle between two directions, for a reclining back, an adjustable arm or a sloped panel.
Radial measures from the centre of a curve to its edge, and diametral measures right across it. Use radial for a fillet or a rounded corner, diametral for a tube, a hole or a round tabletop, since a diameter is what people quote for those.
Ordinate measures positions along an axis from a common origin, which is how a technical drawing marks hole positions along a rail. Useful for mounting patterns where the customer needs to check against something they already own.
Bounding box is automatic. Rather than picking points, you point it at a model and it measures the overall extent. For "will this fit through my door" this is exactly right, and it costs almost nothing to set up.
Setting them up
You choose the points to measure between, and the annotation draws itself. Each one can be styled, so the line weight, colour and text suit your product rather than shouting over it.
Like everything else in Composer, dimensions can be conditional. Show the depth measurement only when the customer has opened the accordion about fitting, or show a different set of dimensions for the wall-mounted version.
Units and honesty
Show the units your customers think in, which means metric or imperial depending on where you sell.
Be careful that what the dimension says matches what you ship. A dimension is a promise, and a customer who measures their alcove against your model and receives something 3 cm wider will be in touch.
Presentation
Dimensions work best switched off by default and revealed when asked for, either through a control the customer toggles or in an accordion about sizing. A product covered in measurement lines the moment it loads looks like a technical drawing rather than something you want to own.
For a workshop, the same is not true. The order PDF on the Pro plan can include technical views, and there dimensions are exactly what is wanted.