Currently, it's possible to change the size of a linked image in JIRA, but this is not documented in the Wiki Markup Helper.

EG: I can link an image with it's size specs;
The unsized image:

!projectavatar.png!

The resized image:

!projectavatar.png|height=250,width=250!

It's also possible to resize an image by percent as well.

But, when I go to the help and click on Images there's no guide on how to do this - https://jira.atlassian.com/secure/WikiRendererHelpAction.jspa?section=images