3-4bb + 1bb per limper is a great starting point and something you can do without really understanding why and still not make too big a mistake.

I will now in brief explore some of the ways you can choose to vary PFR size and what some strengths and weaknesses are.

PFR size varying by hand strength:
Pro: You get more money in with good hands - if opponents are particularly unobservant and will call a 10bb raise with as weak hands as they will call 3bb raises with this is just free money.
You risk less money on weak starting hands - and if you flop big you can always start building a pot after the flop.
Con: You very easily become transparent. AA loses a lot of value if all of your opponents know that you are holding exactly AA and no other hand. They can play perfectly against you.

PFR size varying by position 1:
This version is raising small out of position and big in position
Pro: Since you are generally more likely to win any hands in which you have position you here build big pots while in position that you stand to win, and smaller pots out of position that you are more at risk of losing. This is particularly true against unobservant opponents who don't look at the raise size before deciding whether to call.
Con: You manipulate the opponents range so you end up playing less hands post-flop in position and more hands post-flop out of position.

PFR size varying by position 2:
This version is raising big out of position and small in position
Pro: You play more hands post-flop in position and fewer out of position. This is particularly true against observant opponents who look at the raise size before deciding whether to call.
Con: You play smaller pots in position and bigger pots out of position.

PFR size varying by stack sizes (intro to SPR):
SPR = Stack to Pot Ratio. While such a ratio can be calculated on every street the coverage of SPRs in PNL focuses on it's application between pre-flop and flop. The basic formula is effective stack size divided by pot when the flop hits.
In the below examples I will assume and calculate for one caller.
I can't do an easy pro/con analysis so I'll explain by example.

Let's say you are BTN and your blinds are semi-loose and aggressive - blind stealing is not profitable with any two cards.
The action is folded to you and you have 22
Example 1: The blinds have 100bb and so do you - just do a normal 4bb raise. If you get a caller you are mainly playing for set value and with 24-1 stack odds you are safe to do so.
Example 2: The blinds have 75bb each and you have more - do a 3bb raise. If you get a caller you are mainly playing for set value and with 24-1 stack odds you are safe to do so.

In SPR terminology you are trying to set up an SPR of 13 because your hand plays well with that size SPR.

Same table as above. This time you have AQo
Example 1: 50bb effective stack size. Raise to 6bb - if called you have an SPR of a little less than 4. The hand you are hoping to hit and play post-flop is a TPTK type hand - that hand plays best with an SPR of 4 or a bit less.