When choosing your flowers, you are not making this decision in a vacuum. Although you may have some grandeur vision, you also need to consider the surroundings and how your idea will match or clash with the site of your reception. Some of your vision may not be feasible because of the site so you need to take this into account before you get started and begin to make plans.
Your florist is creating something of a masterpiece in the reception hall so both of you need to remember the background the florist will be working with. For example, if there is a bright wallpaper or a large botanical garden, these will affect how you decorate and require consideration. Or if the area is completely blank, the florist will need to know this too so they know there will need to be a lot of work.
When you first meet with the florist, bring in some kind of photo of the reception site if she has never worked there before. Also, make sure that she visits ahead of time so she can get clearer idea about the size and style of the site to make your flowers just right for the atmosphere.