I understand that both words mean big but when should one be used vs the other?
Both words mean large or great, both figuratively and physically, but they're used in slightly different contexts.
When Talking About Items
Grande is often used when identifying a specific item against smaller ones. Apri il libro grande (not the small one). Grosso is almost never used this way. (While "più grosso" might: Passami il libro più grosso)
Grande is often used to signify large, while grosso means big. Un libro grande has a large format, un libro grosso is bulky, with many pages.
When used before the noun, grande might also mean great, based on context. Un grande libro might be a large book or a really good one. Often both, if making a pun.
Effectively, an item can be grande e grosso if it is both large and bulky.
When Talking About People
All that has been said for items still applies, with some variations.
Grande before the name most often signifies great.
Grande can also mean adult: Una persona grande = an adult. Il fratello grande = the older/oldest brother. Filippo è grande, ormai = Philip has grown up already.
Grande can be about physical height. Guarda che grande è quel tizio = look at how big that guy is.
Grosso is more about width.
Indeed, a guy can be grande e grosso, meaning tall and wide.
As you correctly noted, both grande and grosso can be translated as big. However, although often interchangeable, on a figurative level their meaning can differ quite a bit.
Take for instance the following sentence: there was a big boulder blocking the road. You could translate it as c'era un grosso masso che bloccava la strada or as c'era un grande masso che bloccava la strada. They are both perfectly acceptable.
On the other hand, there is a huge difference between saying era un grosso uomo and era un grande uomo. The first sentence would translate as he was a big man, where big denotes his being robust, corpulent, fat or even muscular. While the second sentence would translate as he was a great man, where great denotes his qualities as a person.
I believe this thread on Wordreference might help you get a better undestanding of the difference between these two words: https://forum.wordreference.com/threads/grande-vs-grosso.615354/?hl=it