Hello,
If you talk about the floor grid, it is not possible.

You can, but only if you place pieces outside of the normal 16 x 16 grid.

I may have been too fast. I understood the size of the base square which is 16mm x 16mm. But effectively the floor includes 16x16 of these squares and it extends automatically when parts are moved further.

Thanks for your replies!

I see now that the grid extends automatically when parts are moved further, thus increasing its size beyond the default 16 x 16.

It looks as though they then have to be positioned within the limits of the extended grid - which is what I want.