Outside of spells like silent image, I'm not sure about lower levels. But the prestige class Veiled Illusionist lets you make yourself look like a human and you gain additional illusionary forms as you level up. It even lets you pilfer spells from the wizard/sorcerer list, but only illusion spells.
Alter Self allows you to take the shape of any small/medium humanoid, and medium creatures are 4-8ft & 60-500lbs, so the largest medium creature can be 8ft & 500lbs. And that's pretty big but still keeps you medium-sized for any mechanical size-based rules. Comparatively, if you wanted to be a large small creature, they are 2-4ft and 8-60lbs, so the largest small creature could be 4ft & 60lbs.
You can do this with the 2nd lvl spell as a Magus, or if you want to save the spell slot, you can get a Hat of Greater Disguise for 12,000g and cast Alter Self essentially at-will.
Also, there's a spell called Fearsome Duplicate but it wouldn't be feasible in combat /shrug.
If you're allowed 3pp feats, the feat Size Doesn't Matter would help your Intimidates vs. larger targets. Ask your DM, he might say yes.
Agile Maneuvers feat would definitely help your CMB for dex-based builds.
Thanks for the suggestions so far, please feel free to keep them coming! I think the Disguise skill or a hat of disguise might be my best options right now. Kuja-goo-goo, I appreciate the Alter Self suggestion but if I polymorph into something in another Size category I lose the bonuses of my Small size. If I'm going to go Dex based with the PC, which is kind of the way I'm thinking with the glaive and Bladed Brush, then using Alter Self to get a size increase means a reduction in Dex and a -1 attack penalty.
I think there's a cloak or robe that lets you store disguises in it. That might be an option, if I can get my GM to let me use Downtime to load said magic item with disguises I prepped ahead of time by taking 20 or something :)
Alter Self can make you into a 4ft 60lb Small creature.... ><;
Which Combat Maneuver are you going to try to specialize in?
Btw, you can get a lot of use out of taking Dirty Fighting and Summon Monster. Dirty Fighting gives you a +4 to CMB vs. any target you're flanking.
I'm not so much focused on the maneuvers RK Go. At low levels, I want to use nets; at level 1 I intended to take the feat Agile Tongue. With that feat I can use my tongue to:
1. lift/take anything weighing up to 5#
2. Use Sleight of Hand from up to 10' away
3. Make Disarm/Steal Combat Maneuvers up to 10' away
4. Deliver melee touch attacks up to 10' awaySo I will be using a high Dex build to make most of my attacks from 10' away from my enemies. As mentioned at low levels I'll debuff foes of Medium size or smaller using my net to deliver Entangled. I'll use the spell Frostbite a couple times per day or more to deliver a small amount of non-lethal damage and also inflict Entangled. At 3rd level I was going to take Enforcer so that my Frostbite also provokes an Intimidate check to Demoralize in order to make my foes Shaken.
Since I won't have a big Str and my Cha is only a 13 I wanted to squeeze as much out of Intimidate as possible. However being Small size I take a -4 penalty trying to intimidate anything of Medium size or bigger. Maybe this build is trying to do too much...
Enforcer won't work for Frostbite, it only affects non-lethal dmg with melee weapons, and Nets don't cause damage. Snag Nets cause damage, but it's 1 piercing dmg (not non-lethal).
And why are you calling me Kuja-goo-goo and RK Go?
If your goal is just to eliminate size penalties. There is the feat Taunt for intimidate.
I also thought about the Belt of Impossible Action, but sadly it doesn't do anything to eliminate penalties it just allows you to make combat maneuver checks in spite of any size differences that exist.
The majestic image spell lets you effectively ignore size penalties to intimidate, but it's not really an in-combat spell.