Singlespeed & Fixed Gear - Quick question about seat post length?

i am about 5' 5'' and i ride a 51cm bianchi pista. i was wondering if i was sitting on the saddle, do i need to be touching both my feet on the ground?

Yes, otherwise falling becomes a comically common occurance. Your feet don't need to both be flat on the ground, but you should be able to balance on the balls of both feet generally.

the best answer is when your pedal is closest to the ground your leg should be almost completely extended, but your knee shouldn't lock out, it really doesn't matter if both feel touch the ground

If you can put both feet flat on the ground while sitting on the saddle, your saddle is way too low (unless we're talking about BMX or downhilling).

Here's a good way to get in the ball-park for correct saddle height. Pedal with your heels on the pedals; your knees should lock out, but your hips should not rock to reach the pedals. That way, when you have the balls of your feet on the pedals, your knees will be slightly bent at full extension. Fine-tune from there.

my feet can't touch while i'm seated. i think it just depends on the person.

if your feet can touch the ground while you're in the saddle, there is something seriously wrong with the fit of your bike.

haha or you have a HUGE inseam

haha or you have a HUGE inseam

That wouldn't matter.

i am about 5' 5'' and i ride a 51cm bianchi pista. i was wondering if i was sitting on the saddle, do i need to be touching both my feet on the ground?

Adjust your saddle height so that you are balancing on your metatarsal-phalangesal-joint. Make sure that your saddle is level when doing this.