Sorry Karen but I Wouldn't touch this one with a barge-poll ! Unless like mentioned b4 you use the minis.
Remember that aswell as the weight making them want to fall out and/or the oasis crumbling if done in a holder,the shape will be horrible coz your sunnies will all be pretty much at top or it will have to be massive. And sunflowers don't last out of water very well so it would have to be made on the day of the wedding! My boss took an order for one and we made him get the very small mini ones. They were hard to get (we got let down twice) and when we did get them we had to rush in another lot at the last minute coz 1st lot went crispy over night. NIGHTMARE!
I would suggest a round hand tied or long hand tied that is staggered and explain that it can be left in water till needed. The long hand tied (think mini tied sheaf style) would be a modern twist on a shower. Could you maybe try and sell it to her like that. Show some pics of long tied bouquets ?
Failing that I'd tell her if its really gotta be a shower then go for yellow mini gerbs with black centers.Still don't last well outa oasis but you would get a much better shape. And I know its a bit naff (but you are talking about a bride who wants a sunny shower tho!) but could you suggest this and maybe compromise by put some good quality artificial mini sunflowers in too ?
Sell it to her as " I know it's not ideal but it's the only way I can guarantee they will last all day out of water".Then if shes got bad taste she'll go for this , if good taste the long traily hand tied. Either way the pressures off for you to make something that is a guaranteed headache .

If you suggest all this then you are giving her lots of alternatives . I really wouldn't do a sunflower shower...
Good luck ! x

PS I am up this early as got baaad hayfever and can't sleep ! Am I the only florist with bad hayfever ???