I found that I can do this using Modify Rooms in online v6.6 of SH3...
This feature does not seem to work in locally installed v6.6 of the programme.

Hi,
i found how to do this.

( I'd been looking for a specific dimensions setting but then realised that the standard icon to inc/dec font size would work)

<t>Hi,<br/>
<br/>
new user here!<br/>
<br/>
Enjoying using SweetHome 6.2 on Linux Mint 20.1.<br/>
<br/>
When I create a room, the default fill is dark grey; how do change this to "no fill" ( I have tried "painting the floor with no fill" but that didn't seem to work.<br/>
<br/>
Can anyone help ...

Hi,

new user here!

Enjoying using SweetHome 6.2 on Linux Mint 20.1.

Cannot see how to increase size of dimension font?

Can anyone help please?