I have some of these shorts and they are long:
http://www.walmart.com/ip/Danskin-No...horts/22227099
These come in a 2x and have the liner built in:
http://www.performancebike.com/bikes...400081__400081 They're not going to be past the knees though.
The only time I've ever fallen off my bike (so far) is when my baggy shorts got tangled up with the seat. My daughter likes to ride with the padded shorts, and she has a pair that has a skirt made over it. It's really cute but she has trouble with the skirt getting caught up in the wheel.