Is it necessary to soak new lifters in oil before installing ?

 YES.....AND PUMP THE PLUNGER UNTIL YOU GET THE AIR OUT

I was reading my repair manual and it states that if you replace the lifters you must replace the cam? Why? and if I do, How do I know what size to put in. Is there an identifing number or mark on the cam to tell me what the size is ?
Thanks 

If the bottoms of the (old)lifters are not concave/scored, or otherwise damaged, you can put new lifters on an old cam. If there is any damage to the lifters, the cam will need to be replaced.
Some aftermarket cams are stamped/engraved on one end to ID them, some are not. If you don't know where the cam came from, or what the specs are on it, it's pretty much a crap shoot.

If I cannot identify what the cam size is, is there a rule of thumb to go by as far as size, so I will not damage any other components ?
This engine is a 427 with .060 over pistons, heads were just done new valves and seals ( not ported ) new hyd lifters, picked up some rockers with roller tips. Would like an aggressive low to mid cam but seriously nothing crazy. This is my 3 day a week fun car, and as much as I want it to be quick I do not want it to break. Also has a 671 blower with very mild boost
Appreciate any insight

 Think there are special cams made for blowers. check out any cam makers web site for info , then call their tech #, they'll give a list of cams#'s suggested for your engine combo