I'd consider going with at least a .72 for F#, as anything below that will be flub. Also F# is definitely doable on 25.5", but it won't sound as clear as say a 27" scale.
Also, you can always unwind the tip of the string to get it through the tuner, which is what alot of us do here.
Check out my band, 1371:
"North Korean women in uniforms waving at the traffic, raining from the sky on platforms, hold by giant umbrellas. Scary... just wait until they invent electricity and traffic lights. Imagine traffic lights falling from the sky and just confusing everyone..." - SD83