A system for selecting a pixel block within an image frame encompassing a plurality of pixel blocks of a predetermined traditional size, for use in motion estimation between successive image frames and embodied in a video encoder for encoding a video data stream, the system comprising: a pixel block processor; characterised in that the pixel block processor is adapted to: select a pixel block of traditional size within said image frame; determine boundaries of an extended pixel block encompassing the selected pixel block and additional pixels defining an overlapped region; derive statistical information for a pixel block of smaller size within the selected pixel block, and statistical information for said additional pixels of said overlapped region, wherein said statistical information is a variance; and select (22, 26) said pixel block of smaller size when said derived statistical information for said additional pixels of said overlapped region has a value less than a product of the derived statistical information of said pixel block of smaller size and a first parameter, wherein said first parameter has a value between zero and one; select (22, 24) said extended pixel block encompassing the selected pixel block and additional pixels defining an overlapped region when said derived statistical information for said additional pixels of said overlapped region has a value greater than a product of the derived statistical information of said pixel block of smaller size and a second parameter, wherein said second parameter has a value greater than one; and select said traditional size block if the derived statistical information for said additional pixels of said overlapped region is greater than the product of the derived statistical information of said pixel block of smaller size and the first parameter and less than the product of the derived statistical information of said pixel block of smaller size and the second parameter. select a pixel block of traditional size within said image frame; determine boundaries of an extended pixel block encompassing the selected pixel block and additional pixels defining an overlapped region; derive statistical information for a pixel block of smaller size within the selected pixel block, and statistical information for said additional pixels of said overlapped region, wherein said statistical information is a variance; and select (22, 26) said pixel block of smaller size when said derived statistical information for said additional pixels of said overlapped region has a value less than a product of the derived statistical information of said pixel block of smaller size and a first parameter, wherein said first parameter has a value between zero and one; select (22, 24) said extended pixel block encompassing the selected pixel block and additional pixels defining an overlapped region when said derived statistical information for said additional pixels of said overlapped region has a value greater than a product of the derived statistical information of said pixel block of smaller size and a second parameter, wherein said second parameter has a value greater than one; and select said traditional size block if the derived statistical information for said additional pixels of said overlapped region is greater than the product of the derived statistical information of said pixel block of smaller size and the first parameter and less than the product of the derived statistical information of said pixel block of smaller size and the second parameter.