The mouse and keyboard question looks to be one that could come up again regularly.
The big question is WHY? Is the real problem making lots of typos, etc, or is it being able to see them and correct them?
Most “large” keyboards are aimed at the visually handicapped. Some only have large fonts, while others have large keys as well (say 1” or 25mm square). Generally speaking, if you have large keys, the keyboard will be the same size as normal, but will have a lot less keys (usually the number keypad and Function keys will be lost). Take a look at the HCL 591-5239 Visually Impaired Person Keyboard and see what I mean. Prices can vary dramatically (buyer beware). Any former touch typist, and a lot of others, will have a problem with a keyboard with large keys – any given key may not be in the same place even if the keys are in the same layout.
Mice are a different question.
First, do not use a mouse with a roller ball underneath – get an optical mouse.
Second, I have found that cheap mice (ASDA, Wilkinsons, or Tesco) work just as well and last just as long as any others. Third, get a mouse with two buttons and a scroll wheel (if it has a USB connection it will also work on a lot of Macs as well). I have an ASDA mouse (£2.99, I think) plugged into my G5 Imac.
If you have the scrollwheel type mouse, and you have FireFox or Opera as your browser, when surfing you can hold down the [ctrl] key and zoom in/out by rolling the wheel to and fro. This works on both PCs and Macs. With the ASDA mouse and several others, you can operate the buttons by pressing with the centre joint of a finger, rather than the tip. This makes it easy to get the bulk of the mouse under the palm, and mouse movements can be made by the arm, rather than the wrist.
So it comes back to the matter of which problem you are trying to solve. It could be that “fumblefingers” can be solved by altering the height of the chair, so that the user’s arms rest on the desk (note that some people sell wrist-rests to do a similar job). A different approach to mice may solve the problem of insensitive finger-tips.
If the user has a PC, it can be worth looking at an interface program called ELDY. It is aimed at the older user and offers both a simple interface, and a social network. Look up http://www.eldy.eu and try it (it is a free download).
Geoff