Marin Frame Measurements (late 80/early 90's) - Guide?

[Beeewu]

Was riding a 19" c to top Team Marin but felt too small, now on a 20.5 c to top Pine Mountain...im 6ft and this is proberly perfect to be honest.

 

[Oceanother]

Hello, I'm 5.7' and I plan to travel to Asia from Europe. My father has an 19' 1996 Stinson hybrid bike and I was wondering if it would be ok to convert it to a travel bIke (butterfly handlebar). Is the size a real drawback for that use? Thank you