This word is, to my eyes, odd, so I went on to have a look at pronunciation of this and came across this thread: http://www.forumwales.com/fwforum/viewtopic.php?f=34&t=8268
People there suggested "pet-REE-al", which makes sense when it seems that at various points in history it has also been written as 'pedrual' or 'petrual', among other things.
Caution - geeky rambling to follow: However, there actually seem to be many other ways of translating 'rectangle'. At least three dictionaries from between 1770-1860 seem to go with 'union-ongl', but this seems to apply to a possible old meaning of rectangle as 'right angle' (I'm thinking of the German 'recht'). I've also come across the rather more recent 'hirsgwâr' (long square - very apt!) on Y Geiriadur Bangor. It seems that the origin of the decidedly weird 'petryal' is a corruption of the Middle Welsh and Early Modern Welsh 'pedryfal' (which has various uses between the 13th century right up to the start of the 19th - see Y Geiriadur Prifysgol Cymru - including the Red Book of Hergest and the Black Book of Carmarthen).
There seems to be some uncertainty about the meaning of the word "petryal." Y Geiriadur Bach translates it as "square". So does the Modern Welsh Dictionary/Geiriadur Cymraeg Cyfoes (Oxfrd University Press), which is generally considered to be a very good dictionary. Both these dictionaries gives it as a masculine noun and as an adjective. "Rectangle" is translated by "pedrongl", plural "pedronglau", feminine. Neither of these dictionaries includes "pedrongl" in the Welsh-to-English section. This would seem to indicate that this is a word which is uncommon in day-to-day speech and is mainly restricted to the language of geometry. Is Duolingo's translation of "petryal" as "rectangle" a mistake? Or is this word commonly used to mean "rectangle"?