Do you have a GitHub account? Best way to do modifications is to set up an account, fork the repository https://github.com/tvheadend/tvheadend and then do modifications there.
If not, or for initial playground, you can go to that repository and clone without needing a GitHub account (git clone https://....). There are some build instructions somewhere on the wiki for dependencies needed to do a local build (so you need dev libraries/headers to build).
There's a Makefile.webui in the repository which combines the .js files in to a .gz file, which is then served, but I just run "sudo make install" instead.
I think on Linux, if you run the tvheadend from within your own build tree, then the ui changes are then picked up automatically; otherwise a "make install". No need to restart tvh if you just make changes to the javascript files, but still need to make/install the gz file.
The widths are in the "width:" sections. Ideally it would be nice to be able to just double click and have all columns resize properly (as in Excel), but that's probably difficult to do.
I found the build instructions:
https://tvheadend.org/projects/tvheadend/wiki/Building