A little late, but the Empire generally plays with Hammer and Anvil tactics. You use very large blocks of state troops to fix enemy units in place while an elite unit of some kind flanks and does the killing.
This means your state troop units(melee ones anyway) need to be larger sized. 40+ if you are talking a normal sized game(2k or so). Smaller you can get away with 25 but as Dave says that is the minimum you want. This large size is needed because, while you can have the mentioned static +9 to combat resolution, there are a lot of potential adversaries which can easily chew through that +9 and you need this block of troops to stay put for a couple rounds without running away.
As for ranged troops, small or minimum sized is the way to go, but you do not want too many of these units. They are very easy for enemy units to outright destroy in one round, overrun, and then rampage through your back lines. They are really just there to do a little chip damage or be a speed bump when you need it.
The ideal combat order for Empire is that the enemy charges their big scary unit into your block of halberdiers. They hold for 1-2 rounds of combat(likely losing, but holding their position), during which you've moved a unit of Demigryph knights or something similar to the flank of the enemy fighting the Halberdiers and you then win the fight in the end. And because the Halberdiers survive the battle thanks to having a lot of bodies they don't give victory points.
This concept really applies to every faction in
WHFB in every edition, but Empire is the one it applies to the most.
State Troops on their own are not going to do damage to the enemy or win the battle. They are enablers for your knights, steamtanks, etc...
A tip on filling out your units with extra models. There is an old trick called "Filler Models" where you can take a larger base(with the same footprint as a number of smaller bases) on which you make a cool diorama or something and put it inside your unit. Empire are on 25mm bases for example. You can use a 50mm base(same size as 4 25mm bases in a square) and make a cool rock or something with 1-2 miniatures on it. It counts as 4 state troops, but only uses 1-2 miniatures. It can be removed as you take casualties.
Sprinkle a few of these through your army and you can save a bit of $$ and make it look cool.

I've seen people use a guy pushing a wagon with some supplies in it, camp follower miniatures, a shrine to Sigmar, an extra flag bearer, etc... Be creative. Especially with the higher model count armies like Empire. Pick up any of the limited edition or weird one-off models and use them in your unit fillers.