The QR codes you can copy in Tracker.ly are 216px x 216px. If you need something larger, for example, for a poster, you can get larger sizes.
QR codes go to a twe.to link so we can track "clicks" on the QR code. If you right-click on the QR code and copy the link, you'll have the same link the QR code goes to.
To get the exact same QR code from Tracker.ly, but larger, use the same Google service we use to generate QR codes, replacing the bold parts with the size and URL you want. (Note, the max size on Google is 547 x 547 pixels.
http://chart.apis.google.com/chart?chs=547x547&cht=qr&chl=http://www.twe.to/code&choe=UTF-8&chld=L|1
To go bigger or fancier, you can actually use any QR generation service to create custom QR codes.
For example, https://www.qrcode-monkey.com let's you go up to 2000px x 2000px, embed images, change colours, and do other entertaining things.