ERT expression for dimensions of boxes, images etc.

[Tobias Hilbricht]

Dear readers of this list,

is it possible in LyX to enter dimensions in terms of ERT expressions
such as \linewidth+\marginparsep+\marginparwidth into the dialogs for
boxes, images (Insert -> Box, Insert -> Image) ?

If I have defined a length in Document -> Settings -> LaTeX-preamble,
what is the LyX way of sizing boxes and images with that length?

Thanks for helpful hints in advance,
yours sincerly
Tobias Hilbricht