Since getting a HiDPI screen, I have been plagued with claws mail merrily doing the right thing with proper emails, but showing HTML emails at a tiny size.
Whilst it doesn’t appear to be a preference you can change in the normal way, there is a zoom variable you can change within the Claws preferences file. Quit claws, and edit ~/.claws-mail/.clawsrc and scroll down through the file until you find the “[Fancy]” section :-
[fancy] enable_images=1 enable_remote_content=1 enable_scripts=0 enable_plugins=0 open_external=1 zoom_level=100 enable_java=0 enable_proxy=0
Change the “zoom_level” to a suitable percentage (such as 200).