Tomas, I agree that increasing roster size globally would kill strat. But only for neutral players. And it would definitely reanimate strat for big alliances.
And here is obvious solution:
Every faction(or maybe every player, but every faction should be better) set preferable roster size for them. Then there is roster size, determined by battle size, which works now. It should be changed so it would calculate minimum and maximum roster sizes depends on battle size.
And final value for roster size = Max(MinSize, Min(team1.size, team2.size, MaxSize))
This would allow small factions and solo players to have good battles with small roster sizes and have full rosters even against giants, and big alliances would have huge exciting battles.