A while back, Marketing Sherpa shared a case study (membership required) for furniture retailer Black Forest Decor. "Making the
logo search box bigger" boosted conversion by 20% and revenue by a staggering 84%.
Black Forest Decor still has a modest sized search box when you compare it to the trend of super-wide boxes we see on the likes of Linens N Things, Overstock, Sears and PC Mall:
Black Forest Decor
Linens N Things
Certainly you don't want a box so small your customers have to search for the search box! And enlarging your search box, making it more prominent in design, styling, "Search" button or placement, leads to higher search box use, less site abandonment and higher conversion. That makes sense.
But is there such thing as too big? I believe the answer is "yes" when the search box is so big it looks like a banner ad, or otherwise fades into the background of the other elements on your page. It's possible that's the case with the sites above.
As always, it's best to test a bunch of sizes and placements before you decide which search box is right for your site.