Do you man RAM (real memory) size or hard disk memory size?
Also, do you mean their physical size, or their size in megabytes / gigabytes?
If you are buying a laptop then I assume you mean size in megabytes / gigabytes.
There are two main sizes you need to consider:
RAM (real memory) size and hard disk size.
RAM (real) memory is measured in gigabytes and nowadays it is best to get one with at least 2Gb. If you can get one with 3Gb or 4Gb so much the better, but unless you do complex computer work you are unlikely to use more than 2Gb so dont pay extra for 3Gb or 4Gb.
Hard disk size is also measured in Gigabytes and nowadays disks are huge compared with a few years ago. Usual sizes are 250Gb, 320Gb, 500Gb, 1000 Gb.
Note a 1000Gb disk is also known as 1Tb (Terrabyte).
Try to buy the largest you can, though most people doing normal computing work will rarely fill a 250Gb disk (unless you download lots of films and music).
Cant help on the ipad as I know little of the internals but the same principles of RAM and hard disk still apply, though sizes may bge different (possible smaller).
If I have answered the wrong "question" then let me know.