There's two ways to go with this.
If watching it grow is the goal, then use a bigger jar (like mayonnaise or pickle jar) so the roots are not constrained. I have herd of people keeping avocado growing until top is over 24" tall with multiple leaves. avocado can draw nutrients from the big pit so it can grow for quite a while in water.
HOWEVER, if growing it to be a house plant is the goal, then it's best to plant it while the root is short so as not to be damaged in the planting process. Also, water roots are different from soil roots and too much upper growth will become difficult to support during the transition when planted.
I usually start avocado seeds in 6" deep containers and 1qt deli container works well, but then it needs to be Uppotted rather quickly. Someone who is not familiar with the process may want to start with a 6" deep by 6" wide or 8" deep by 8" wide container (at least 1/2 gallon container -- you could re-purpose a plastic tub or recycle a plastic jug by cutting off the top for added flair to the experiment but make sure to make plenty of drainage holes).
Avocado needs well draining potting mix, so I add about 1/4 sand and aquarium gravel or shredded mulch into the mix, thoroughly mixing with rainwater or de-chlorinated water until holding a handful forms a damp ball that crumbles (think home made pie crust). Sand and gravel helps to weigh down the container if using plastic. I like to add compost to the mix, but commercial potting mix will probably contain some fertilizer.
They grow better in clay pots. If using plastic container, make sure it has plenty of drainage holes (decorative pots often have completely inadequate drainage holes).
After loosely filling the container to the top, thump it down on a surface to make the potting mix settle, then add some more mix up to 1/2 from the top rim. Poke a hole in the middle for the root, then snuggle the avocado seed/pit gently down until the bottom 1/3 is under the soil and upper 2/3 is sticking out. BE SURE TO HOLD GENTLY since the pit is already splitting.
Water gently with -- a water bottle with holes punched in the cap with a nail works really well as a waterer. Rain water or de-chlorinated water is best. If all went well, you will probabky need to give about a cup of water until water runs out from the drainage holes (the container should be sitting in a drip tray to catch the water). If the water in the drip tray is not absorbed back in 1/2 hour, dump it out. POTTED AVOCADO SHOULD NEVER SIT IN WATER because it is prone to root rot (reason why clay pot is better).
If the soil was loose and washer down, exposing more of the pit, you should add a little more so bottom 1/3 is covered.
Hope that was enough.
-- oops! Forgot the most important part -- Avocado from seed is usually cross pollinated so it won't grow fruit like the original avocado. In any case it takes years -- like 10-15 -- for it to grow to fruit bearing size, and it's difficult to impossible to achieve this in colder areas where the tree needs to be potted and brought inside during the winter months. That said, it is a relatively easy to grow plant and can become an interesting houseplant if given plenty of light and allowed to vacation outside from spring to fall.