I am a 5 8'' 135lb f beginning snowboarder, what is the best board and right lenth/size for me

Most companies have boards that they gear toward the beginner rider. They're mostly all-mountain boards & if they're from a good company they should last u a few years. Some womens boards that I've heard r good for beginners r the Burton Feather & Ride Solace. My friend has the Solace & it really improved her riding over what she was doing on rental boards. So I'd recommend that one. Here is last year is model on sale for $199 at REI: http://rei.com/product/756574

As far as size I'd go for something between 148-152 cm.

Hey! Im 5'8'' 135lbs too! what a quinky dink! lol anyway l just got my new board. l got a rome lo fi 150 (pretty much an all mountain board maybe geared a little bit more towards jumps) so l think around there would be perfect for you. go to http://frostyrider.com/tips/size-guide.htm & put in all ur info & it will give u an appropriate size range as well as some suggestions. it gave me 150 - 153 as a good range for me :)
some brands i've heard good things about - rome, k2, nitro, gnu, libtech, forum, burton, ride.. & many more but theres a few.

good luck buying
have fun riding!!!!