JRubyArt is actively being developed, ruby-processing should be considered to be deprecated and is no-longer actively maintained. The only possible justification to prefer to use ruby-processing is to be compatible with processing-2.2.1. So since January 2017 table below has been modified to address what you need to know, now that you have decided to switch from ruby-processing to JRubyArt

feature JRubyArt-1.7 JRubyArt-2.6
binary k9 k9
run opts -r, -w, -l -r, -w, -l
jruby version (2.5.7) (2.6.8)
jdk version jdk8 jdk11+
config PROCESSING_ROOT not required

The settings method was introduced to vanilla processing since processing-3.0. However this is hidden for users of the processing ide but required by Eclipse users. The settings method is where size belongs or full_screen, also you should set smooth and pixel_density here. Retina users can make use of their hi-dpi display by setting pixel_density(2), NB: size should be first line of settings, and if used pixel_density(2) should be next.

In JRubyArt use the data_path wrapper to return the absolute path for the data folder see here (this means --nojruby flag is obsolete since even shader sketches should now run with an installed jruby). Set JRUBY: false in config.yml to use jruby-complete instead of an installed jruby (crucial if you haven’t installed jruby on your system).