commuter sizing question

OK...so I am going to join the commuting by bicycle world soon and looking for the perfect bike. I have mapped out a few routes (variety is the spice...and different paths for didfferent weather)

1st - the Blue Ridge Parkway: this would involve the Honey to drop me off at a specific location and i would be able to ride the roadie allllllll the rest of the way:) This would be on nicer days and of course when he is up for a short morning drive (the road from home to drop off point is far too narrow and sketchy during morning rush hour - only about 2 miles from home...then about 10 miles on bike)

2nd - zip through the neighborhood on paved road to the bus stop...ride bus through high traffic area...pedal on up a steep hill to work (remember the bike will be on the front of the bus encouraging me not to get a very pricey bike)

3rd - and most exciting if you ask me...cruise the singletrack behind the neighborhood to the bus stop...ride the bus...and pedal on up the steep hill

Sooo...1st route is good cause i got the roadie. the second and third are the question...I decided i am going to get either a cross bike or a light hardtail mtbike so i can go on and off road my issue is the size...because i may use this off road should i be more concerned with the stand-over height than if it were strictly paved roads? I found a perfect hardtail mtb but the standover was higher than my road bike by at least 3 inches...now this only bothers me because i may be off roadin it some. Because of this i am looking at an extra-small which just seems too small...
I am 5'1" with a 28 inseam

Yay for you!!!!

I would try each route on the weekend, and get to know it well. Know which one is scariest for traffic, which one takes the longest, which one will get funky in the funky weather, and then any given day you can make your decision. I have a long route and 2 short routes; the shortest is on busier streets but if it's early it's not a problem.
Does the parkway get a lot of commuter/rush hour traffic or do most cars keep to town? If there isn't much traffic that's where I"d lean towards but the windiness and narrowness are real disadvantages when having to coexist with cars. (I'm feeling all sensitive 'cause I just went to a "bicycle treatment" seminar and listened to the highway engineers discuss it.)