The pgf manual is the best source for this. It's just very comprehensive, so it's not easy to find. Have a look at
15.7 Establishing a Bounding Box in the
pgf manual.
TikZ normally makes the picture as big as necessary so that its content fits, however you can specify the canvas size, i.e. the bounding box, yourself, using the command
\useasboundingbox
.
Btw. you don't always need to search the internet. Very probably the pgf manual is already on your computer, as documentation usually is. Just type "texdoc tikz" or "texdoc pgf" at the command prompt. You can also use
texdoc.net as a service.
Stefan