Master of Orion/General battle tactics

Ship Size
Generally speaking, ships are made for her pleasure, the bigger the better.

Part of this is because it is easier to get whole kills on small ships. Any hit on a small ship is practically guaranteed to kill it at any point in the game. If the ship dies, its ability to attack is obviously gone. It is, then, easier for the enemy to reduce your ability to damage them by quickly killing a lot of your firepower.

Larger ships can usually take many many hits before they die so it is hard to reduce your ability to counter-attack if you are using large and huge ship designs.

Additionally, the huge ship designs are practically built to use automated repair and advanced damage control. They have the ability to use all the best defensive systems at the same time and to reduce the damage they take massively. The automated repair is based on max hit points, so if they can't do 15% of the hit points of this ship in one turn then they will never kill it. If they can only do 20% damage to it, it will take like 17 turns to defeat the ship. That is a lot of time for your firepower to be going to work. You can retreat the ship when it is close to death and inflict many casualties and have none of your own.

It is very hard to pull this off with tiny and medium ship designs.

Scouts as blockers
In the beginning of the game, weaponless scout ships can be very useful in combat.

If an enemy ship has no weapons, it will always retreat if it is the only thing in space combat on its side. This happens very often in the early game when all the ships in the game are scout ships and colony ships.

Thus if you can get to a planet first and leave a scout in orbit over it, you can keep enemies from getting info on the planet. They won't know the environment and planet size if they can never end a turn with a ship over the planet (until they get Advanced Space Scanner).

This is useful because it allows you to make their colony ships turn around and go home and buy yourself time to get colony ships to the planet first. They will have to come back with ships that have weapons in order to get your scout to retreat. Sometimes they might send as many as 3 colony ships without an escort and all of them will have to return home. Other times they will send an escort with the first run. Still, it is worth it to try this, especially on really good worlds, if not every world near you. The less information the enemy has, the better.

Usually, if you have ships over a planet, the enemy will try to expand somewhere else rather than fight you for it. It will usually only fight you for it if it scouted it first and then you showed up to keep bouncing its' colony ship or if it has run out of places to expand elsewhere. If it has run out of places to expand elsewhere, you are guaranteed to experience fighting over these planets if there is nowhere else for the computer to expand.

Invasions
Never, if you can possibly avoid it, let an enemy fleet end the turn with a fleet in orbit over one of your colonies.

At even the easiest game settings the computer will try to invade with colonists if it can do this. In the hard and impossible settings, they are guaranteed to try to invade if this happens.

It is very important to avoid this at all costs. It is not worth trying to setup a trap like this, it is better to not even try to do something like that. Doing so is just going to be taking risks that will only very rarely pay off.