Welcome here, Johann!
Unfortunately there is no simple way to add parts length to LPub for parts that doesn't support it: it's defined by compilation in LPub executable
So apart from trying to make changes to the source and recompile the whole thing, the only trick I found was to temporarily change description of parts for which I need to show length, and name them to another part that does show length. Eg. here change "Technic Axle Flexible 19" into "Technic Axle 19". To do that you need to change the 1st line in 32235.dat file in your LDraw\Parts folder.
Unfortunately there is no simple way to add parts length to LPub for parts that doesn't support it: it's defined by compilation in LPub executable
So apart from trying to make changes to the source and recompile the whole thing, the only trick I found was to temporarily change description of parts for which I need to show length, and name them to another part that does show length. Eg. here change "Technic Axle Flexible 19" into "Technic Axle 19". To do that you need to change the 1st line in 32235.dat file in your LDraw\Parts folder.