Depends on what she considers "more". Do you know how girthy her current ones are? Either the actual size or the product names of one or two would give a better idea of what range to look for.
Personally, I'm currently a massive fan of http://www.lovehoney.co.uk/product.cfm?p=20872 as not only is it decently chunky, but the asymmetrical shape makes it feel bigger without over-stretching.
On the budgetier side http://www.lovehoney.co.uk/product.cfm?p=16364