Hi,
Welcome to the Fusion 360 Community!
Once you click okay to that dialog box you can't get that box back again to edit dimensions manually. You have to use the edit form button (which can be found in the Modify menu) in order to change the desired shape and size. A good workflow for the sculpt environment is to choose a decent sized body to start off with (in your case a box) with not a lot of faces. The analogy I like to say is that its like a ball of clay. You start off with a lump that is the general rough shape and size, then by pressing and pulling you can narrow down the shape and size even further. In order to get more intricate and complex with your "ball of clay" you will need to add more faces in certain areas that you want to make more complex. This can be done by using the "Insert Edge" or "Subdivide" command in the Modify menu.
I actually created a screencast for another using experiencing a similar issue. I hope this clears it up for you!
http://autode.sk/1IBZJWT
If you need more clarification, let me know. I'm more than happy to help! If these suggestions answered your question, feel free to mark this answer as a solution. That way other users experiencing a similar issue can benefit from this post.
Also let me know if you would like any links to more tutorials that I've found to be really helpful in learning Fusion 360!
Thanks,
James Youmatz
Product Insights Specialist for Fusion 360, Simulation, Generative Design