Luxembourg & Liechtenstein used to be the smallest nations to compete in the Euros that has now expanded to 55 countries.
Formed in 1954, UEFA originally consisted of 29 nations.
The break ups of the Soviet Union & Yugoslavia expanded the competitor list from 2 to 18 alone (Armenia, Azerbaijan, Belarus, Bosnia & Herzegovina, Croatia, Estonia, Georgia, Kazakhstan, Kosovo, Latvia, Lithuania, Moldova, Montenegro, North Macedonia, Russia, Serbia, Slovenia & Ukraine).
In addition micro nations Faroe Islands, San Marino, Andorra and Gibraltar were welcomed into the fold.
Could we see Nagorno Karabakh, Transnistria, Greenland & Monaco one day?
Luxembourg

I have visited the Royal Duchy twice, during inter rail tours in ‘87 & ‘89.
We stayed in youth hostels on both occasions and had a pleasant if not entirely memorable time.
Liechtenstein

Tiny Liechtenstein was the last country that I visited for the first time, my trip being in February 2019.
I travelled by train and bus from Zurich, staying in a small chalet style hotel in Triesenberg high up in the mountains.

An interesting fact..
A EUFA founder member in 1954 was Saarland which today is a State in Germany but in 1954 was an occupied Rhine State that was under post war French control. Saarland is/was more or less the same size as Luxembourg.
In the same year FIFA World Cup Finals competitors included West Germany who by a curious twist of fate had qualified for the finals by beating Saarland!
Despite the objections of France who wanted to retain the occupied territory on account of its coal and mineral wealth, Saarland was reunited with West Germany in 1957 and so was therefore no longer entitled to independent membership of EUFA.
Not a lot of people know that.
Fascinating as ever!