Placement, not that I know of... Size and depth, yes. I would just make it easy and do it on the front of your magwell.
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There's no size requirement for anything except SN, which is usually reserved for F1 cans and 80% lowers.
"Conspicuously placed", as the statute is worded, would not include under the pistol grip (if you're talking about the tab that gets covered when the grip is installed).
Engrave it where most people place it...On the blank side of the Mag well.
My preference has been inside the trigger guard using the smallest size font that's practical for the engraver. I suppose you could get to a size that would arguably be too small to pass the " conspicuousness" label. As to location, I've always only engraved stripped uppers so no disassembly was required. I guess I wouldn't really care where it was engraved otherwise, although I suppose reselling the lower would be easier if it wasn't too conspicuous.
As to box 4H, I just put my full name, city, state, without abbreviation to make the NFA examiner happy. There's no requirement that the engraving match 4H. The engraving only has to meet the law. Most of my SBRs only have my first initial, last name, city and abbreviated state. I'm confident that the ATF will be fine with that, in the extraordinarily unlikely event I will ever run across an ATF field agent out on NFA patrol.
I recently had this done. It's 1/16" in size.
http://i67.tinypic.com/u0th0.jpg
Thanks for the information I think I'm just going to go ahead and put it by the trigger