Why are my Word 2013 page size options different to other Word 2013 page size options?

The loptions in my laptop Word 2013 page size drop down are very different from those on my desktop at work. My home machine has fewer, and e.g. is missing the A5 paper size. I can only guess this is because of different versions, but I installed using the same key, and even the same install file, if I recall correctly.

My other line of suspicion is that because my laptop runs an expensive mobile data connection, it gets its Windws Updates less frequently and has missed one.

Please check the settings for the printers that you have installed at the different locations. You will find that the work printer has many more pre-defined paper sizes. It is this that Office and other Windows apps pick up.