A unique scale legend is included on the map in the form of four large lines of text running down the middle of the map between England and Wales. The text says that Briain is 800 Roman miles long and 300 miles wide. (Anglia habet in longitudine miliaria DCCC ... In Latitudine vero CCC).
The majority of Scotland is shown as an island, connected to the rest of Britain by Sterling Bridge, which is labeled 'Estriuelin pons' on the map
The sense that Scotland is a separate island from England & Wales is reinforced by the place-name 'Scocia Ultramarina'. This describes Scotland as beyond the sea.
Scotland is also divided from the rest of Britain by Hadrian's Wall. The wall is shown on the map stretching from Carlisle (carleolum) to Wallsend (wallesend). The text describing the wall reads 'The wall which divides the Angles and the Picts' (murus diuidens Anglos et pictos).
Scotland is also divided from England by a second Roman wall, the Antonine Wall. This is shown on the map by a single line labeled 'murus scotorum' (Scots Wall). The Antonine Wall was a turf fortification on stone foundations, built by the Romans across what is now the Central Belt of Scotland, between the Firth of Forth and the Firth of Clyde.
Wales is described on the map as "A mountainous and half-marshy land of busy and productive but bellicose men descended from the Trojan Brutus".
In medieval British history Brutus, or Brute of Troy, the legendary descendant of the Trojan hero Aeneas, is seen as the eponymous founder and first king of Britain. When Matthew Paris says that the Welsh are 'descended from the Trojan Brutus' he presumably is suggesting that they have a direct line back to the very first Britains (descended from Brutus and his fellow Trojans).
This isn't the only role that myth plays in Paris' descriptipns of Wales. On the map the Welsh town now called Carmarthen is labeled as 'caermerdin id est civitas Merlini'. Translated from Latin this reads 'Caermerdin, i.e. the city of Merlin'.
You can continue to explore the map on your own. Most of the place-names on the map are interactive. If you click on them you can read the modern English translation.
I've translated around 80% of the place-names and Latin text on the map. However I haven't been able to read all of Matthew Paris' map labels. So my apologies for the untranslated towns on the map.
If you are intrigued about the torn off half of Britain then you might want to refer to one of the other three Mathew Paris maps of Britain. The one owned by the British Library includes Southern England.
If you want to explore more interactive maps then why not head over to Maps Mania.