You'll need to download some kind of image editing program that supports transparency if you don't have one already, first off. GIMP should be fine for this purpose. It's free and it'll get the job done.
Question is, do you need to make the image itself larger while maintaining the transparency, or do you just want to make the transparent area larger?
If it's the first one, you'll need to go to the Image menu > Scale Image > set the dimensions to whatever you need. The entire picture itself should be enlarged.
(Don't uncheck the "maintain aspect ratio" checkbox/the little chain inbetween the image size dimensions unless you want it to get all stretched and awkward looking)
If you just need the transparent area around the image itself to be larger, go Image > Canvas Size > set the dimensions to whatever you need. You can drag the image to where it needs to be on the canvas with the move tool.
In order to make sure the transparency stays as-is, you'll need to go File > Export As > whatever file type you need, though I'd go with PNG if you don't need anything specific i.e. GIF or JPG or whatnot.
stupid example of exported png here too
