when i try to open a picture into photoshop, it is very small, how can i fix this?

How do you have your render options set?? 

If you want a larger render, you have to first set up your render dimensions.  At the top, click on the Render selection, then click on Render Settings.  Set up the size of your render, then make sure to select "Render to: New Window". 
A thing to remember, the larger the image, the longer the render is going to take ;) 

i did, it doesnt work, and my render is so cool!

Did you also change the pixel size of your render?

Are you hitting File>Save or File>Save last render (after rendering)? The former saves the scene, with a 91 pixels square thumbnail image for the Content palette - it's the latter you want to save your images.

If you open your render>render settings and click on the presets bar under deminsions and select "custom" you can type in your heighth and width sizes (I use 1280x1024 which is a 4.5 ratio and fills my screen completely. After you type them in, click accept at the bottom and the viewport will show you the areas that will be included in your render.