Hi, Rebecca,
I've found that the AA road atlas of Great Britain and Ireland will be all that you need. It's in an 11 x 14 inch format. They are available at the petrol stations at most supermarkets, also at bookshops, TI centres, and newsagents. It shouldn't cost more than 10 pounds.
The one thing that the AA road atlas is missing is detailed maps of the cities, but you most likely will not need that, as there aren't that many major cities in Scotland. The RAC used to print a really nice spiral bound road atlas, which included the cities. I found that the places which I was looking for were always in the spiral!
I've never needed a GPS (SatNav) in Scotland, as the maps are so detailed. An Ordnance Survey map will come in handy if you plan to do a lot of remote hiking, but most routes are well signposted.
Happy travels!
Mike (Auchterless)