It’s not big. Understandable given the history of Rome and what must be buried beneath the city. There are 2 lines, A and B, and several ticket options ranging from 120 minutes to 3 days. And the station had ticket machines and a ticket office.
For our first trip we chose the cheapest, 100 minutes at €1.50 each, because we were going 3 stops and figured we could get there and back in that time. €3 is a bargain.
At the ticket machine the smallest note I had was €20. I spent several minutes going through the process several times: select language, select ticket option, select number of passengers, insert €20 note, retrieve €20 note from reject slot, select language etc etc. Eventually I read the message on the screen “maximum note value €5”. So I turned to the ticket office. And read the sign: “NO TICKETS NO CHANGE”.
Back up to the square for an ice cream (it was hot today), then once more to the metro with my newly acquired €5 note. We were off!
Visited The Spanish Steps (much more impressive looking down rather than up and evidence I climbed them)
Then back to the metro before our 100 minutes expired.
It was at the automatic ticket barrier when I discovered the second sentence on the Valid For 100 Minutes ticket: Single Journey Only. Fortunately our earlier ice cream left us with enough change so buying a ticket at the ticket machine went much smoother.