Fulgur8's page

If I am reading the official FAQ properly, I believe you will not be able to combine multiple actual size changes to your character. But if somebody else could confirm if this is the correct interpretation...

Size increases and effective size increases: How does damage work if I have various effects that change my actual size, my effective size, and my damage dice?
As per the rules on size changes, size changes do not stack, so if you have multiple size changing effects (for instance an effect that increases your size by one step and another that increases your size by two steps), only the largest applies. The same is true of effective size increases (which includes “deal damage as if they were one size category larger than they actually are,” “your damage die type increases by one step,” and similar language). They don’t stack with each other, just take the biggest one. However, you can have one of each and they do work together (for example, enlarge person increasing your actual size to Large and a bashing shield increasing your shield’s effective size by two steps, for a total of 2d6 damage).

Link Paizo FAQ

Personally, I tend to try and follow RAW. But that doesn't mean GM's always have to get the short end of the stick. There usually is a way for the GM to accomplish this without needing to make custom rules. For this case, let me chime in some ways that a GM could "adjust" the battlefield whenever the upcoming battle could be too easy for the players.

First, it is important to know the specifics of the 5-foot step:

Take 5-Foot Step

You can only take a 5-foot-step if your movement isn’t hampered by difficult terrain or darkness.

So for example, if the battlefield floor is difficult terrain, it is not possible to take a 5-foot step.

Difficult Terrain

Difficult terrain, such as heavy undergrowth, broken ground, or steep stairs, hampers movement. Each square of difficult terrain counts as 2 squares of movement. Each diagonal move into a difficult terrain square counts as 3 squares. You can’t run or charge across difficult terrain. Flying and incorporeal creatures are not hampered by difficult terrain.

Difficult terrain is a game term. A GM can easily come up with original ways of describing the floor and can say add at the end of the description it is "difficult terrain". Or the GM could also let players find out for themselves when they first attempt to move through it.

For the players side, there are a few ways around this. Here are two ways they could:
-Find a way to fly
-Take the Nimble Moves feat

As a GM, I like to reward a player that finds ways to follow a character concept. If a player were to spend a feat to take a 5-foot step in difficult terrain, then he should be able to. But this also means that only that character, and not the entire party would be able to do it.