The phrase "large forehead" simply means a bigger or taller forehead compared to normal. It refers to the size of someone's forehead being larger than what is typically seen.
Sentences with «large forehead»
At the time I considered myself «the man» because I had adorned my extraordinarily large forehead with an overly gelled middle parting in which the strands either side curled up towards each other. (faith.org.uk)
These haircuts will aim to create an illusion of a shorter face because they cover up large foreheads typically seen on long faces. (stylecraze.com)
I typically don't love how they look on me, I have a very large forehead... however this tortoise headband just called out to me. (littlejstyle.com)