If I put. 8 GB SD card in my device,(at the moment it is 2 GB)will it have any effect on the way it runs?If I load say the maps of the whole of Europe will have a detrimental effect to its speed and/or performance?Many thanks for ant input.

Your device will not load all maps, only the map from where you are and the DEM file if present. That DEM file is a memory eater.
I have never experienced a problem with larger SD's on my devices.

The size of the SD card is not usually a problem as long as your device's specifications says that it's OK. It's the class number that can affect the transfer speed of information. The higher the class number the faster the transfer but don't expect supersonic speed.

The more content that is present, potentially the slower Primo (or iGo) will load as all files are briefly scanned during startup for authentication by licenses, etc. However, in most cases this is a very minimal amount of extra time, and the size of the SD Card is not a factor at all.