The table you bought when it was only you and the kids is now the table where your mother eats two nights a week. Something has to give, and it is usually the elbow room. The good news is that you can figure out the fit in an afternoon with a tape measure, a pencil, and the chairs pulled out.
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Count the real seats
Start with the number on the label, then count the chairs that actually fit around the table without bumping the next person. Include any chair your mother uses, and any walker or wheeled chair she needs to get to the table. If the walker does not fit between the chairs, you have learned something before the first dinner, not during it.
Measure the path, not just the table
Pull each chair out and sit in it. Check how much room is behind the chair for someone to stand up and sit down. Leave enough space that she can get in and out without a tug from someone else. Check the seat height of the chair she uses against the table height, because a chair that is too low can make the table feel like a wall.
Check the leaf and the storage
If the table has a leaf, find it, see whether it fits the table, and decide who puts it in and who takes it out. A leaf that lives in the garage and needs two people to install is a leaf that will not go in on a Wednesday. If there is no leaf, compare the options: a second small table in the same room, a rolling cart that holds the overflow, or a plan that moves dinner to the kitchen counter on the nights the table is full.
Give dinner the table first
The homework pile and the mail will always try to take the table. Give the table to dinner, and give homework a tray or a desk. Put a basket by the door for the backpacks, so the table is clear when the plates come out. A table that works for four people often works for five, but only if the clutter has a home that is not the middle of the table.
You are not trying to buy a bigger table. You are trying to make the one you have fit the family you have now, with room for everyone to sit, stand, and pass the potatoes without a fuss.