Buying a rug can be kinda overwhelming, there are a lot of factors to consider. How to choose the right rug size for your space is difficult, especially when rugs are notoriously kinda expensive and therefore intimidating. In general people buy rugs that are too small because 5×7 and 6x9s are so much cheaper than 8x10s or 9x12s, but its often going to make the whole space look chopped up and accidental so in a way you’ve just wasted that money. Measure the size of your seating area before shopping and get the closest size up from that. If your budget is tight and your room is small you can have a smaller rug that floats in front of all the furniture this is especially ok if your sofa is backed up against the wall you don’t need to waste the rug real estate under the sofa if you can’t even see that its there. But if your furniture is floating or if your room is big, get a larger rug or else it will make the room look smaller, chopped up and totally out of proportion. One of the main functions of a rug is to pull the room together, not chop it up.
Orient the rug to the room if you have a long rug, orient it length wise. If you have more of a chubby fat room, orient it horizontally. This is not always the case so its a very nuanced situation. Often if you have a really long and skinny room then you might have two different seating areas and two rugs that are oriented with those seating areas, but the rugs need to cover up a certain amount of real estate on the floor in proportion to the room, so if you are orienting the rug to your sofa, not your room then often you are cutting off huge portions of the room and again, making it look smaller. So when in doubt, go with the orientation of the room. Square rugs are for square rooms. That is all. Round rugs are for foyers/entries or kids room/family rooms that are more playful.
If you have a rug that is too small but you love it, then buy a large sisal that fits the whole seating area and layer your favorite rug on top . Sisal, jutes, seagrass rugs are cheapish so buying a 9×12 is more doable than a 9×12 antique kilim. Make sure that your rug is has at least 6-10″ on either side of the sofa. This is crucial. If the width of the rug is the same as the width of the sofa then you are in trouble – it will look too small and create a bowling alley effect for the property. If you have to choose which legs should be on, then choose the ones that are the least obvious. That is, if your sofa is low and closer to the wall then you are less likely to see under it and reveal the fact that its not fully on the rug.