Hi,
If you need to get the wheelchair under the table, and are checking clearances for the legs, then you will need to measure her.
You will need to measure from the floor, to the highest part of her which needs to go under the table.
For manual wheelchair users, you would have to measure from the floor, to the top of their thigh, however, if the user has armrests on their wheelchair, you would have to measure to the top of the armrests.
If the user has an electric wheelchair, you would have to measure from the floor, to the top of the joystick controller, and add a couple of inches for clearence.
The depth of the table can be estimated by getting the user to pull up to a wall, so their toes are touching the wall. Then measure from their stomach to the wall. This measurement will give you their leg clearance. In the area where their legs go, there should be no crossbars which could catch their feet.
As for the table width, just make it big enough to get her chair under, plus extra width in case two people in wheelchairs want to sit at the table at the same time.
If she can use a joystick, then the chances are, she will be able to use a tracker ball such as the Kensington Turbo mouse. This requires minimum hand movement.
I've added a photo of my computer desk, which I designed, and had made to give you further idea's
Regards
Simon.