It all depends, really. I mean, if you are just a member of a fireteam, you will have a different breifing than the squad leader, which is different than the platoon leader's breifing, which is different than the company commander's breifing, etc. But basically they all follow the SMEAC form, and that is most of the required info. Each section could be as short as a sentence if you wish, so you don't have to just fill up space.
One thing about breifings is that they can be used to "set the stage", or sorta tell the story, i.e. what is going on in this war at the moment. It puts the mission in perspective, which really helps the enjoyment for some people, like myself.
Here's a quick example. Let's say the mission is an attack on St. Pierre. 1st platoon, Charlie company will attack the town, and the player will lead 1st squad of that platoon. This order would be sent to all the squad leaders in 1st platoon:
Situation: Russian forces are occupying the strategic town of St. Pierre. They are using the port to bring supplies and reinforcements to the heavily defended southern Everon penninsula. Batallion wants Charlie company to secure the town to cut off the Russian logistics tail.
Intel says there is a platoon of infantry and 4 T80 tanks defending the town. There still may be civillians in the town as well. There is also an AA outpost north of town, where two squads of infantry are defending 2 Shilkas.
Mission: 1st platoon will assault the town from the west. 2nd platoon will attack the Russian outpost north of town, as a diversion to draw reinforcements away from their well-prepared defenses in St. Pierre. 3rd platoon will be waiting east of town in reserve. 1st platoon must eliminate all defenders of the town, and destroy the Russian supply ships that are docked there.
Execution: At 0545 2nd platoon will attack the outpost, with artillery and mortar support. On the command from PAPA BEAR, 1st platoon will commence the assault on St. Pierre. 2nd squad will attack the northern edge of town, 1st squad will proceed through the center, and 3rd squad will attack along the road in the south of town. Batallion has ordered that no artillery support will be available to 1st platoon, to minimize the damage to the town and civillian casulties. However, Cobra gunship support will be available once the northern outpost is secured.
Admin/Logistics: Each squad will carry 3 LAW launchers. 1st squad will carry three additional Carl Gustav launchers. 2nd squad will have a sniper team attatched to them. Medics will be present in all three squads.
Command & Signal: Captain Demer (PAPA BEAR) will coordinate the operation from the hill east of town. Gysgt. Barron (the player) (Alpha 1) will lead 1st platoon in the assault, from 1st squad. Sgt. Hansen (Yankee 1) will lead 2nd platoon in the assault on the AA outpost. If either attack is in need of reinforcement, a flare will signal for 3rd platoon to assist.
1st squad is designated Alpha, 2nd squad is Bravo, and 3rd squad is Charlie. 2nd platoon will be Yankee, and 3rd platoon will be Zulu.
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Now, that would be a lot better with links to markers on the map. I could have added even more to each section, but that would be a lot right there. So much of the info might not really help, but it gives the player with an active imagination lots to think about. Even if there really aren't units on the map in those other places (2nd and 3rd squad), it is nice to imagine that they are. But after writing all that down, you might actually want to put those units there.
Writing a breifing like this might also help you flesh the operation out in your own head, and give you ideas for the mission. From the above breifing, you might decide that if the player fires a flare, 3rd platoon will help him in the assault on town. HOWEVER, you might also make the ruskies do a counter-attack from the east, so if 3rd platoon moves into the town, they will leave that flank undefended. 2nd platoon might also run into trouble, and not be able to take the northern outpost. But they can't call on 3rd platoon to help out, since the player already asked for 3rd's help. So the AA outpost won't be taken, and the player won't be able to call on airsupport.