Moving into a larger home is all about timing.  You need to know, approximately, what you are going to get for your existing home, and what you want to spend on your new home.
We recommend getting a pre-approval based on conservative numbers for the sale of your home.  If it turns out you sell for more, that's great!  The important thing to know is that, if you sell for your lowest acceptable price, you will be able to afford to buy the new home.  Getting a pre-approval as early as possible (ie; before you list your current home) is important.