Following the manual, section “Advanced user preferences”, issuing this in a Scheme session before generating the figure worked
(set-preference "plugins:embedded postscript width" "0.9par")
Maybe with a little bit of work one can write a code that issues this command, taking as argument the figure width, as one presses a key combination. I do not see yet a good place where to insert the argument.
To write the code, you can take a look at Set Pasted Image to 0.8par and centered (you have to select the correct tree).
Also, in the Executable environment the Return key has a Scheme definition that makes it activate the Python code I think. I would look for that definition in the TeXmacs Scheme code and see if you can integrate those functions in your own code that both activates the Python code and sets the figure width.