Hi Alice,

If you wanted to describe 'a big opportunity' then you would use the adjective 'grande':

Una oportunidad grande. Grande meaning 'big'. 

If however you wanted to describe 'a great opportunity' you would use the adjective 'gran':

Una gran oportunidad. Gran meaning 'great'.

Gran: 

- Placed before a noun means 'great', 'fantastic'. It's more about describing the qualities of a person or thing.

Grande: 

-Placed after a noun refers to size or age meaning big, large, old or grown-up.

Oh and 'gran' can go before both a masculine and feminine noun.

 I hope this helps a little.

My Grammar book (by Butt and Benjamin) confirms what Clara says - but in the context that, when it means 'big', 'grande' is restricting or differentiating the noun so it needs to follow it [as explained in the Kwiziq lesson on noun/adjective ordering]. And when it precedes the noun [here, translated as a 'great opportunity'; i.e., one which is very welcome] then it gets truncated to 'gran', even when feminine [but remains 'grandes' in the plural; i.e. 'great opportunities' would be 'grandes oportunidades'].