what are they for, and do they have to be a certain size?

 

venturi effect and to cause a vaccum i think, also i would presume it seriously improves the way the air is drawn down the flutes

 

they are there to try and send the inlet pulses(which travel up the inlet track) back towards the inlet itself!

size and shape matters,,, alot

 

doesnt matter what size then so long as thee all the same and the inlet is tapered?

 

oh didnt see that

 

the bell mouth actually increases the velocity of the air , and smooths the airflow out, the ratio of bellmouth diameter to lenth will dictate your engine charicteristics, ie long narrow bellmouths will generally give better low down response while wide shortones will give better top end power

 

to elaborate on RWD Cossie wil's answer: The shape allows a higher reynolds number (related to flow coeff) different lengths and radii can be used to tune the flow for different rpm. As will says long for low down and short for top end.