In 4.0 the menu items are now displayed in a smaller font size. I have tried to change the font size but not seem to make a difference.
Any idea on how to change this ?
Hi Harry,
Tools > Options > View gives a set of options to change some menu parameters including Font Size from 80% to 130%. Also to choose your ICON set. Worth spending a little time having a little look.
Note that this does not seem to be available in version 6.1.5.2
In additions to changing menu parameters Installing libreoffice-gtk would integrate your menu font size and style with your system settings. That would make libreoffice more appealing. Hope this tip helps someone. 
Excellent, thank you !. This solution was perfect for me. Libreoffice menu fonts and toolbar icons always looked badly sized and spaced when running on a KDE desktop, ie Kubuntu or KDE Neon. Installing libreoffice-gtk made a huge improvement. Much appreciated, much better than the other suggestion above about changing font sizes icon set.
now try: libreoffice-gtk2. Note that this works using Debian/LXDE as well (inheriting openbox/lxde theme fonts/sizing).