This is likely a simple question for an experienced player/DM, but I am wondering if very large creatures can take larger "steps" without provoking attacks of opportunity in battle in DnD v3.5. Additionally, does it seem vastly unfair or mechanically imbalanced to make this a house rule if it is disallowed in the PHB/DMG?
There is no rule stating that large creatures get larger "steps".
The major issue with a greater than 5' step houserule is that it allows large monsters to step back further than a PC can step forward, effectively preventing melee characters from making full attack actions. If you don't use this tactic often, then it wouldn't be a problem.
Also be aware that there are many ways for PCs to enlarge themselves, allowing them to take advantage of large steps as well. This is very useful and quite potent, but is by no means game breaking.