You can supply the .hide
class to the container of the modal to hide it, which is the default bootstrap method of hiding modals. As to the size, you can again supply your own class with your custom width and height to the container and it should take up on it, the way you're adding it inline now works fine as well, just make sure to add it to the main container.
So all in all your modal container should look like this:
<div class="modal hide fade custom-size" id="myModal"> ... </div>
Note: Added the .fade
class to add the transition fade/slide effects, you can remove it if you like if you don't want those effects.
And as for the modal poping up as soon as you load your page all you have to do to remove that behavior is not call the modal at all as you are now, that way the modal will only popup when you click on action or "Launch Modal" button.
Here is a cleaned up fiddle, you had some non-valid styles like float:center
(i wish that one existed): http://jsfiddle.net/zkzuM/2/, full page demo: http://jsfiddle.net/zkzuM/2/show/