For a vinyl pool, typically not a huge price jump to make it bigger. If yoru yard can fit it, look to see price bump to increase to 36 or 40 in length. I have 18x40, love it.
Pool shape, I like an oval, or rounded with rectangle with at least 2 foot radius corners. This allows for better water flow. Rectangles can have poor circulation in the corners, and the corners prove a bit more difficult to brush/vacuum.
Pool placement. Go outside now, and track the yard shadows, and path of sun throughout the day. If possible, place the pool in a position that will give full sun from 10am to 4pm. This will optimize high water temp.
Heater, thnk about it you need one or not. Then learn about gas or electric. We choose to prewire for electric and make pad big enough for it, but have never bought one. Decided that it was not needed for our usage and sun availability.
As /u/Winger 03 said, decking. Try to preplan where you might put your pool and pool furniture. Take a rope or extention cord, and trace a perimeter of the pool. Then go out and place furniture in teh grass (use boxes or whatever else if you dont have any furniture) along the pool. Use another rope or extention cord to trace a perimeter of concrete decking all the way around the pool. Make sure that people have room to walk around furniture without having to go in the grass. Envision someone sitting on the decking, with feet in water. Can you walk around them easily without going into the grass?
Decking part 2, where are people going to put their stuff? I have been in way too many pools that were not preplanned, and everyones pool bags end up piled up and in the way, or getting wet.
Decking part 3, remember that you will need to cut the grass still. Dont leave little islands of grass that you cant get the mower into. Dont leave strips of grass to narrow have a mower go over. And no hard corners, round the corners as they are more of a pain to cut.
Decking part 4, go buy some little flags, and/or cans of yard marking spray paint. Dont be scared to try to mark the edges you have traced with a rope. This will allow you to live with teh shape/layout for a few days, making sure you like it.
Hardscape. Lots of pool builders want to simply put in a 16x32 rectangle, 3 foot of concrete around 3 sides, 6 feet on the shallow end, and call it a day. But a great pool includes not just the pool, but the decking/landscape/etc around the pool. Think of amazing hollywood pools you might see on tv. Sure the infinity edge pool is awesome, but what tends to be awesome, even if you dont realize itif the way that the surrounding area is just as amazing. COvered porches, tvs, lighting, plants, etc.
Pool cover. You are in Kansas City, you will need to have a winter cover. This could be $1k plus, and should last a decade or longer.
Costs. Your pool project cost will balloon quickly if you dont preplan all of the indirect costs. Sure there is the pool, and the decking, but you have other costs. Fencing around the pool. Electrician costs. Permits. Pool cover. Will your home owners insurance increase rates due to pool (some do some dont). Yard improvements such as necessary french drains, etc. Once the pool is done, who is responsible for the final grade of the yard? The yard will need to be landscaped and/or have grass seed planted. Will you need top soil brought in. Pay attention to your PB quote, and what is and is not included. You actual cost can quickly get to be 25% to 50% more that the pool itself.
A liner shoudl last 7 years minimum, and might last 10+. Just budget $1500 to replace and install the liner. Typically the liner will fade in color first,ultimatley ending up pure white if it lasts long enough. Then it will fail. The most likley failure is that on the sunniest side of the pool, along the water level, you will end up with a horizontal tear. A lot of people will r\note that at the end of a season, the liner is getting faded, they will schedule a liner replacement in the fall or early spring before it ever tears. The worst thing that can happen is having yoru liner tear in May/June. Pool workers are slammed, and you might end up with a 6+ week lead time to order a liner, have it delivered, and have someone have time ot install it. Before you know it, you can almost lose an entire season.
Oh, and opening and closing yoru pool every year. Opening is pretty easy, most homeowners shoudl be able to do this on their own. CLosing is a bit more challenging, but still can be DIY. You need to drain the lines properly and winterize equipment to prevent freeze damage in winter. Most pool techs might chrage 200 to open and the same to close.
Hope this list helps.