I work in the area of Lean Portfolio Management ( We work with SAFe). We use T- Shirt sizing extensively at the portfolio to get a feel for the medium term investment and capacity allocations. Your point about no XXL is spot on. This is the equivalent of saying I’ll take the whole ART for 3 PIs please. Not helpful.
My guidance at the portfolio level is:
1. Ensure your Epics are clearly linked to a short to medium term (1-3 PIs) objectives / KRs.
2. Make them small enough to give meaningful insight to the plan, ie. Have enough granularity that you can plot some outcomes that are linked to objectives.
3. Use your Solution Architects in planning and setting T-Shirt sizes. As a dev would with Story points - in other words, leverage their experience.
Recently we have been running experiments to try and calibrate the Epic T-shirt with feature volumes and velocity. Early days.
On Jira, not sure about out of the box, but it didn’t take much for our admin to configure our Portfolio Epic issue types with T-Shirt sizing fields.