hello

i found the bike i will buy for now until i decide to invest in a light road bike.
i have found Trek hybrid bike - not sure what is the type, which I like, but as it is on sale they only have a frame size 17.5". I am 174cm tall with normal maybe slightly long legs and the seller is convincing me that it is too small for me. i tried to sit on it and I don't find it that small. However, i am unexperienced in buying bikes and i'm not sure.
i can't take it for a try for a few days.
Should i order a bigger one (he is suggesting 19.5") or is this suitable? he should have different sizes in a month or so.
any advice?

thanks

Nina