Mon Jan 7th, 2019 at 03:23:31 PM EST
Since extending A50 might possibly come up once or twice in the next few weeks this link to the legal decisions on EP seat distribution may be handy:
However, in the event that the United Kingdom is still a Member State of the Union at the beginning of the 2019-2024 parliamentary term, the number of representatives in the European Parliament per Member State taking up office shall be the one provided for in Article 3 of the European Council Decision 2013/312/EU1 until the withdrawal of the United Kingdom from the Union becomes legally effective.
European Council Decision 2013/312/EU of 28 June 2013 establishing the composition of the European Parliament (OJ L 181, 29.6.2013, p. 57).
Once the United Kingdom's withdrawal from the Union becomes legally effective, the number of representatives in the European Parliament elected in each Member State shall be the one provided for in paragraph 1 of this Article.
All representatives in the European Parliament who fill the additional seats resulting from the difference between the number of seats allocated in the first and second subparagraphs shall take up their seats in the European Parliament at the same time.
There is no legal reason a long extension can’t be done, “just” political ones.