To change the font size for gtk apps in slackel openbox use: Openbox configuration manager (obconf) , Customize Look and Feel (lxappearance) and Desktop preferences (pcmanfm --desktop-pref) utilities from "Settings" menu.
To change the font size of qt apps e.g. Dashboard Slackel install qt5ct package. Add the export QT_QPA_PLATFORMTHEME=qt5ct in the end of /etc/profile.d/qt5.sh file. Then run qt5ct and set font size to 12px
For modern laptops of resolution 1920x1080 set font size 12 or 14px.
fonts for gtk apps on user-settings-slackel-openbox are 14px.
For Firefox set zoom to 120% or 133% from Edit -> Settings menu on Firefox.
For lxterminal set the font size to 14 from lxterminal Edit-> Settings menu.