Many nations had "anti-tank" rifles from the end of WWI to about 1941, since tanks were rather thin skinned and an HMG to 20mm sized round could do some worthwhile damage. As a point of comparison, the most common anti-tank cannon at the start of WWII was 37mm.
Practical problems of size, weight and recoil limited how large an anti tank rifle could actually be. The Russian 20mm gun was probably about the maximum size such a weapon could get and still be "man portable" (Even a "New Soviet Man" would have difficulty humping that thing across the field).
By 1941, it was obvious that tank design was evolving to bigger, faster and better protected vehicles that were invulnerable to the sort of rounds any conceivable anti tank rifle could fire. Crew served anti-tank cannons were also growing at an astounding pace, moving from 37mm at the beginning of the war to truly monstrous 122mm designs by the wars end. Fortunately for the gunners, German 122 mm or British "32 pounders" were only experimental, and never reached the field.
The return of giant sized "anti-material" rifles was sparked in the mid 1980's or so with the introduction of the Barret "light .50" rifle. Although it is possible to kill a man at 2400+ m with a .50 cal rifle, the main use is to disable soft or semi soft targets, for example punching holes in parked aircraft, fuel tanks and the like.
I have held a Boys anti tank rifle, and it is not very well balanced or easy to use. For some quick links, try:
http://www.phoenixcommand.com/Boys55atr.htm
http://www.winterwar.com/Weapons/FinAT/FINantitank2.htm#boys
http://www.geocities.com/Augusta/8172/panzerfaust6.htm (a German counterpart)
http://www.rt66.com/~korteng/SmallArms/boysat.htm (WWII Russian)
Hope this helps