In the UK, the size and number of constituencies is determined by the Boundary Commissions for England, Scotland, Wales and Northern Ireland. The number of constituencies is currently under review and it was planned to reduce the 650 constituencies to just 600. The review will be implemented with Parliamentary approval by the General Election of 2022.
The aim of the commission is for each constituency to consist of roughly 70k people. The largest of these is Isle of Wight at 108,804. The smallest is Na h-Eileanan an Iar at 21,769. The largest constituency in the UK, geographically, is Ross, Skye and Lochaber – which is about 12,000 sq km. The smallest is Islington North, whose area is just 727 hectares (Labour Leader, Jeremy Corbyn’s constituency).
Most constituencies have undergone substantial changes over the years. Lincoln however has remained unchanged since 1265. It won the title of oldest constituency when the city of York was split in two in 2010.