Image sizes in spun

Hi there,
i am new to all this, but have used WP before and never had any problems changing my image sizes within posts.
I am using SPUN as a theme which is great, but when i add content to my post, the images are all small and i cant find a way to edit them to be larger.
Is this something i should do in Photoshop before hand to make them all the same size?
I am using the site as an online portfolio so having nice big images is important.
Any help would be great,
thanks!