The first and foremost selection of a type of canvas to choose from various options.There are options like iphone, A4 and so on. For logos which type is best to choose from? Another question, what should be the ideal measurements of a logo that would fit on the website, facebook profile picture, twitter profile picture?
It's best to select a standard page size when you create your logo. As Illustrator art is vector based, you can scale a logo to any size without loss in quality. So, for instance if you scale it to billboard size, you can always save it in a large sized document at that time.
Hope this helps!