Quick answer
grande after the noun = big in size. Plural grandes.
gran before a singular noun of either gender = great, important. This shortened form is the apocope, and it only happens in the singular.
In the plural the form is grandes in both positions: grandes hombres (great) versus hombres grandes (large).
Side by side
Memory trick
Before a singular noun it shortens to gran and means great. After the noun it stays grande and means big.
Typical mistakes
Vivimos en una grande casa cerca del parque.
Vivimos en una casa grande cerca del parque.
Size goes after the noun. Before a singular noun the form would also have to shorten to gran, which would change the meaning to "great house".
Mi abuelo fue un grande hombre.
Mi abuelo fue un gran hombre.
Before a singular noun the adjective apocopates to gran. The full form there is simply ungrammatical.
Check yourself
Fill in: "Ha sido una ___ oportunidad para nosotros."