Currently the right selection panel accommodates three rows of eight icons, i.e. a maximum of 24. Some civs can already build 24 different structures, which means no additional structures can be enabled (e.g. D2801). This patch tweaks the gui to add a fourth row, so up to 32 can be displayed:
This adds more flexibility for future additions and mods, as well as for players who like to group different structures and units together (Ctrl+1 etc.).
While at it, indentation of the gui/session/selection_panels_right/*.xml files is also corrected.
[EDIT] Phabricator can have difficulty handling images, so here is the added png in case it doesn't work for you:
D2875 is an alternative approach, shrinking icons instead of increasing the panel size. See also https://wildfiregames.com/forum/index.php?/topic/28525-larger-panel-or-smaller-icons/&tab=comments#comment-401080