Ryobi doesn't identify the diameters of their fuel lines, so you're going to have to eyeball and match up against the broken pieces you kept when you took the string trimmer apart. Most common size is 3/32 x 3/16 x 3/64wall. Also used are .080 x .130 x .030wall and .117 x .211 x .047wall.

I would recommend Tygon brand fuel line (F-4040-A). It's more expensive but resistant to ethanol (but not bullet-proof). Instead of a year with generic vinyl, Tygon might last three years. However, the .117 x .211 is generally used in Stihl and is NOT Tygon formulation. Don't know how robust it is.

As for power, the 21cc in the SRM-225 will outperform the 31cc Ryobi. The Echo spins 11,000rpm at WOT and produces maximum power at 9,500rpm. The Ryobi develops 9,500rpm at WOT and max power at 7,300rpm, I've got the same engine in my Echo SRM-2100 and it clearly outperforms the 30cc to 32cc half-crank engines.

Thanks for the reply after further thinking I think I'll just get myself a new trimmer. I was worried that a 21cc trimmer like the Echo SRM-225 would be a downgrade in power compared to the 31cc Ryobi but your stats dispel that.

Think I'll pick up the SRM-225 should overall be better I would think.

Had the same issue. I took my old fuel line to autozone and matched it up to the stuff they had in stock. $1.50 / 10 mins to replace and it was good as new. Also cheaper than a new trimmer.