So I am training for a triathlon, I am biking 5 miles a day 5 days each week. I also do a chest-biceps workout rotating every other day 5 days a week. I figure that presses, incline presses, curls, flyes, chin ups, and push ups are enough for upper body and biking 5 miles is enough for lower body. I usually do 30 min. of weight training and 30 min. of biking, which tolls to about 5 miles. I don't squat anymore, but used to max out at 225, my diet is good with little junk food and lots of grains, nuts, fruits, protein, etc. I usually get probably 7-9 hours of sleep a night on average. Will biking build my leg muscles, and not shrink them down?
Your leg muscles will adapt to the workload placed on them, so it's not really possible to definitively say whether or not your legs will get bigger.
If the effort you are putting out is more than you did squatting, then yes, your legs will grow. If it is the same, then your legs will stay about the same size, and if the bike effort is less, they will shrink some.
According to Wikipedia,
The main muscles at work in cycling are the quadriceps and hamstrings in the upper leg, and the gastrocnemius and soleus in the calf. These muscles contract in a sequence that creates the pedaling action. The quadriceps and hamstrings do most of the work when you ride a bicycle.
Assuming you do not know too much about how muscles work, after you workout, your body repairs or replaces damaged muscle fibers. This is done through a cellular process where it muscle fibers fuse together to form new muscle strands or myofibrils. These repaired myofibrils increase in thickness and # to initiate muscle hypertrophy (growth). Obviously in order to gain more muscle, you will need to be eating on a caloric surplus, ESPECIALLY since your biking regime is very rigorous and calorie taxing. However your legs will still get bigger as long as you eat more calories than your burning, sleeping well, and consuming enough protein along with your training.