When we use big words we disconnect with the people we are trying to communicate with, as they tend to wonder what the big word means instead of listening to what we are saying.
Don't assume either that the person who you are communicating with is English either, not everyone who speaks English has English as their mother tongue, so keep things simple from the start and if they communicate in a higher English then you can always step up your English. The stepping up of your English if necessary is less obvious than if we step down because we have started too high.
Think before you speak, keep the way you communicate as simple as possible and the way you speak will be more authentic, than if you were trying to impress the other person by how much you know.
Remember they don't care how much you know anyway or how eloquent you are, they only care how much you care.