Hello!
Totally agree with Luca´s post.
Once you get them, market sizing exercises are actually easy, methodology is very very repetitive. However, until you reach that point, they seem scary.
The only way to get comfortable with them is actually to practice as much as possible, both theorical isolated problems -you can find plenty in this forum- and integrated market sizing exersises embebed in cases.
PM me if at a point you want a coach´s help with it, I have material for both types.
Hope it helps!
Cheers,
Clara