Are you using the Undead's special "Morrigan's Call" spell? (adds +1 growth per death shard your empire controls and has built a temple on.) Of course, if you have no Death shards it won't help...
Alternatives include building consulates for outposts (expensive and not immediate, but doable) and fighting lots of wars (population goes to the nearest city, so it won't help much in the "back rows".) For the very early game, it can be worth "farming" outlaw/mercenary sites as they add to your population when killed.
Also - you probably don't really need to grow beyond size 3 for a fortress (which gives you the strike garrison upgrade to offset your init penalty.)
For your military - for anything that sees melee, use the female units with spirit trait (1/2 physical damage reduction.) Very strong. Also, make good use of the "Curse of the Undead" trait, especially for expensive units; if feasible, grow the unit by bashing in some weaker targets before they see veteran opponents. Finally - remember to build lots of units, even if subpar - they cost you no upkeep and look scary to the AI.