I'd say 4 bars would be a minimum for each section. As for how long - as long as you like. 16 or 32 bars will probably the limit you can take if you're only using 3 chords.
Working out which chords are for each function is easy if you know the scale. Bb major is
Bb C D Eb F G A
I II iii IV V vi vii
Uppercase letters are major chords, lower case ones are minor chords. You know the notes by simply taking your root (i.e IV - so Eb) and skipping a note. This takes us to G. Skip again and it takes us to the beginning - Bb. Eb G and Bb.
Your chord progression is up to you. Starting on I is usually a good idea - although you could have an intro which starts on V which then releases to I for a good effect.
Don't forget not to just use your chords in root position. In fact you'd want to actively NOT do this - it will sound very jumpy, especially from I to V or vice versa. So instead of going Bb D F to F A C you could go Bb D F to C F A. The each likes to hear steps or common notes, not jumps. As long as a chord has the same notes it *is* the same chord, no matter what order the notes are in.
Call and response is pretty easy to understand. Take 'twinkle twinkle little star'. The melody is Bb Bb F F G G F (call) Eb Eb D D C C Bb (answer). The call goes up, the answer goes down and finishes where the call started (although it doesn't have to do this). A call will sound 'unfinished', and an answer should usually sound 'finished'. Note that symmetry should not really be followed TOO closely - a perfectly symmetrical melody will actually sounds quite unnatural and forced. Use broad strokes and loosely follow it.
Melody means 'tune'. Harmony means 'other stuff'. If you think of a gospel choir, you'll often have a leader singing the tune, the melody, and all the other singers will pick different pitches but often sing with the same rhythm. They are harmonising. Again, harmony doesn't have to use the same rhythm.
With the piano the right hand (or the top note of the right hand if it is playing lots of notes) is almost always the melody.
A very very basic melody and harmony might be
RH (notes) Bb C D Eb******Eb D C Bb
LH (chord) Bb******Eb******Eb*******Bb
Some other things to bear in mind - try to take an idea and develop it. Don't just play it once and then do something else. The ear likes to hear repetition.
Think about texture. Maybe you'll want a bit that is very busy and a bit that is very still. A good way to do this is to take a section and double the length of all the notes (this is called augmentation).