Haven't heard of taller lifters, except for the fact that roller-type lifters are taller than flat tappet.
Longer (or shorter) pushrods are to get the rocker arm geometry correct so that the sweep of the rocker tip across the tip of the valve stem is correct. Reduces friction but more importantly it assures the valve is opening and closing as far and fast as it should at all points in the cam's lift range.
Often, longer valves are installed to accomodate taller valve springs in higher lifte applications and that is one common reason why longer pushrods are required to get rocker arm geometry back where it needs to be. Also, aftermaket heads often move the valvetrain gear around in subtle ways vs. stock locations and that can throw off the rocker arm geometry and again require different length pushrods to put it back where it needs to be. There are other things that can throw it off as well.
It's the reason why serious builders always check rocker arm geometry and select appropriate length pushrods to match. There are many variables that come into play and checking for proper pushrod length on every combo is the only way to be sure you've got it dead-on.
