I've operated on a few model railroads and very few I've seen operate like the real thing. Almost all are operated as staging yards, which means the trains are placed on the tracks and the trains depart from and return to the yard. There is no switching or classifying of cars at the yard.
Most of the real yards I've worked have a set of arrival/departure tracks (large yards even have separate yards for trains arriving and trains departing) and a set of classification tracks. The longest tracks are for arriving and departing trains, while the shorter tracks are better for classifying the inbound trains into outbound blocks. One or more blocks of cars can be "doubled over" to another block and then placed on a departure track for the outbound train. I've worked yards as small as four tracks and even these use the arrival/departure operation scheme, though the classification isn't very involved and usually happens between two tracks.
Most prototype yards are double ended, meaning there is a ladder on each end. If you have the room, go with a double ended yard. If you can't do that, don't worry. Just make sure you can get inbound power off at least one of your longer tracks and make that your arrival track. You can also use the double ended tracks for making head-end pickups, if you choose to have your trains "work" at the yard as they pass by.
If you're going to work the yard at all, that is, classify cars in it, make sure you have a lead you can work on without fouling the main. This is the single biggest requirement and it is the most often overlooked thing on model yards (I've had to deal with the misery of switching on a prototype yard where there is no lead separate from the main - you get to bug the dispatcher the entire time, not fun). If you have the room, make the lead at least as long as your longest classification track plus whatever you use for yard power. This way you don't have to foul the main to pull your classification tracks and shove your outbound trains over to the departure tracks.
Aside from considering how much room you have for a layout, you need to consider how many people will be operating the layout with you. You'll need to make controls easily accessible to the person operating each section of the yard. If you have a panel for controlling switches, put it nearest where the person switching the yard will stand. You'll also need to accommodate operators bringing through freights or passenger trains past the yard as well as operators bringing inbound trains into the yard and taking trains out. They will need to get around the person switching on the lead and possibly coordinate movements with that person. Make comfort of the people operating here a bigger priority than squeezing as much track as possible into the room.
Your yard should be sized according to the trains you'll handle there. If you only have five industries on the entire layout and room to spot only 12 total cars, you don't need much of a yard for classifying cars for your local. If you have other yards on the layout, and hundreds of cars to operate, you might need a bigger yard. Once a prototype yard is over half full, it becomes difficult to operate and reaches the undesirable state we call "plugged." Try to keep enough room to move, build and break down your trains. Size the yard based on the number of trains you think you'll operate. Realistically you'll only be able to operate about half what you think, so if the yard seems small at first, just make your switchlists harder for your operators!
If space (length) is really a concern, look at ways to build a compound yard ladder to shorten the distance it takes up. Short yards are still common on the prototype - often I work out of one that was built in the 40s. I don't think there's a single train I've seen yet that fit in one track. Sometimes you have to use three tracks to yard the longest trains. You might also consider using crossovers near one end of a stub ended yard, which has the effect of a ladder at that end of the yard without eliminating the storage space you get from those stub tracks beyond where the ladder would be.
Excuse the poor graphic (away from home and I don't have a graphics program or a mouse with me), but here's a picture showing what I'm talking about:
The red line is the main. The long black line beside it is the yard lead. The top layout shows a standard ladder, with a group of parallel tracks coming off switches placed end to end. The second layout shows a compound ladder, with a switch coming off a switch. This increases the angle you can build your ladder and allows you to shorten the space the ladder takes. The bottom layout is a butterfly style with two ladders. Mirror the ladder onto the stub end and you have a double ended yard. Mix and match the different types of ladders to your liking or based on the space you have on your layout. Or use crossovers on some or all of the tracks to create a ladder at the other end. The possibilities are limited only by your imagination.