I'd probably start with a 4x6 then upgrade to something like a 6x8 or maybe an 8x10 if I got a pair instead of just one, something big enough to do half water and half land.
And yeah, I just got a new red tail, she's 10 years old, 8.5 ft, and weighs more than my labradoodle...and she just bit her tongue earlier...ugh it bled everywhere and she was never handled at her old home so its hard to help her.
AWWW She bit her own tongue? Good grief, is she okay? I know I'd be a little nervous, knowing she wasn't handled and such. Is she head shy? I'd assume yes, at least a little? and being that big, I'm sure she knows she's got some weight to throw around...You have your hands full!