I've now worked with two models of RDS in the past couple months.
One was the RDS built-into the Encresso 300SS. Using Preferences-->Publishing--Text Command TCP, you can use legacy commands to send PS_TEXT= with whatever you want up to 64 characters except that the unit I worked with cutoff everything after 31 characters. Or you can use ARTIST=<a>||TITLE=<t>||DURATION=<d> which populates those fields in PS1 and then you can have PS1 use <item.title> by <item.artist>. But even here the RDS in this transmitter still cutoff the artist and title to 31 characters. Also sending blanks for PS_TEXT= to it would turn PS1 off with no way to turn it back on except the web interface. Oddly, the 31 character limits were not there through the web interface and they would accept the proper 64 characters in the RT and PS spec for RDS. The manufacturer is aware of the issues and hopefully they will fix them. It's a good value transmitter otherwise.
Just this past week I got to work with the Audemat FMB-80. No problems with limiting the number of characters. I found it best to set it up to use UDP and use the ARTIST=<a>\rTITLE=<t>\rDURATION=<d>\r. This thing scrolls very intelligently. One thing that can trip people up is having the max length limiting your text command so that the duration isn't sent sometimes if the artist and title are longer and when the duration isn't sent you get no result. Duration must be the last part of the string.
I'm making changes and additions to Radiologik DJ for the 2017.2.1 to add \n line feeds, < s > for seconds and <s-XX> to subtract seconds from the duration so the RDS can cycle through the default x seconds before the tracks ends. Also I added the option to never publish default publishing to RDS since this unit has its own default. I've also removed the max length as I think any units out there will limit what they want anyway. You should have enough control limiting the <a>, <t>, and <l> if you think you need to truncate.
I have permanent access to the FMB-80 so I will know it well and can help others with that.