Need more info - do you mean a temporary summer visitor, as in summer bedding, in which case, what does 'have size' mean - a foot high and wide? 5 feet? or are we talking about a permanent plant such as a shrub 6 foot high?

Lots more info needed - soil type, aspect (South/East etc) size of border, proximity to fences/walls. What you mean by size; whether you are working to a colour scheme/gardening style.

A statement plant should be something that makes a statement throughout the year, not just something that flowers in summer. And unless you want to spend a lot of money on a fully grown specimen it is unlikely to make make much of a statement for the next couple of years (or more). Take a look at websites such as Big Plant Nursery or Architectural Plants (both in Sussex). It's also a useful to look at what your neighbours have in their gardens - it can give you an idea as to what might grow well or what you don't want (from a taste/competition point of view)