Probably just being dense, but....

[morrisman]

Looking at buying a road bike as my hybrid is fat and slow (bit like me really); so I'm on the Decathlon site and, for example, am looking at the Triban 7 and the sizing options are:

51/20,08IN
54/21,26IN
57/22,44IN
60/23,62IN

What does it mean?

Clueless of Wendover!

 

[ianrauk]

These are the top tube length in CM's and Inches.

Scroll down on THIS link to show you the fit sizes.

 

[Boon 51]

If you can try and get to sit on one as manufacturer's sizes differ a little.

 

[Onthedrops]

How tall are you and what's your inside leg measurement? As Boon 51 said, the best option is to pay your nearest Decathlon a visit and try one out in the flesh.
The T7 is becoming very popular with many people buying them.