change image size automatically

I have tried to find anything about the topic without luck.
I use Sandbox.

I am wondering about if it is possible to put size for the image that is uploaded in a post?

I don´t mean the existing sizes that wp already have to choose between when uploading a picture.

I would like it to be so that I don´t have to rememeber what size I have at the div (#left in my case). If it´s possible to somewhere put a automatically size for the images that are uploaded to the posts?
Is this even possible or is it just wishthinking…?

Thanx in advance!

My site.