I would brand a replacement heifer before she was bred or wait all the way until after she calves.  Best age is 2-4 months.  When we'd buy preg. heifers we always waited until branding the calves to do the cows.  Hate to put that kind of stress on them while they are carrying.  2"-3" for calves works fine, double that for a cow..  I wouldn't use an "I" with a top and bottom cross bar.  Should easily be able to make any of them out of one iron.  An "F" needs a relief between the top and bottom cross pieces and maybe one on the top cross as well.