Placing different size posts in specific order?

I’m using get_post_class to style certain category posts in different sizes. ‘Big’ posts get a full width of the page, ‘medium’ gets two thirds and ‘small’ gets a third. However because I can’t control the loop’s post order as it is coded now, the posts do not fit nicely in the container column without any white space.

For example, if the loop calls a ‘small’ post and then a ‘big’ post, each post will take up one row because they are both block elements floated to the left that cannot go out of the bounds of the container div. Is there any way to fill the empty space with a post automatically, or otherwise dictate the post loop order to be ‘big’ then ‘medium’ then ‘small’?

Edit: shoot, I must’ve posted this in the wrong forum. Can someone move this to the themes forum?