Does anyone have some tips or thoughts about how to balance the encounter?

I have not found anything in the corebook on how to balance an encounter.

I will have between 3 and 6 players in my group, as we are old and have families, the group will differ in size from session to session.

What I need is to have the ability to size up or down the encounter according to the amount of players.

I've found that this is mostly a thing of feeling rather than numbers, and that situational factors and surroundings make really influence an encounter. As such the only three advice I can come up with are; modify number of enemies depending on number of players, introduce NPCs that can give some assistence and fill out the squad when the number of players are low, or introduce factors that improves the chances for either side, like advantage in cover to one side, possibility to suprise one side, enemies being wounded from an earlier fight, fog or darkness if one side or another would have an advantage in a situation like that.

One advice I would give is however that if in doubt, make the encounter on the weaker side. Its in my opinion better to let a fight be easier than intended than make an unintended party wipe. And really, if the fight is going to easy, you can always let enemy reinfocements arrive or something. Its essentially always possible to make a fight harder after it has begun, its more much difficult in my opinion to make a fighter easier without it being obvious to the players that the GM is messing around.

Balance the encounter? What it is?

In Warhammer one is not supposed to have "balanced encounters". One is supposed to think "how not to bite more than what can chew me". And pray.