I have uploaded my logo I would like to increase the size of it as it looks very small and unclear given there is so much space around it.
Just make the picture bigger in your picture editing program. Manager imports the picture to the size the original picture is. So make the picture picture and crop the blank space around it. You might want to save it as a png to eliminate background colours which can cause lots of space around the picture.
There is also a maximum size coded into the sales invoice template. This can be modified, but requires knowledge of HTML.