Greetings!!

I have been working with Category, Post, Comment like scenario, and I always have given them

a larger datatype that is bigint.

However practically there is no need to choose such large value for the above mentioned scenario but I wanna know that

1.) is there any harm in choosing such larger datatype,

2.) is it slow to process that large datatype in comparision to smaller datatype e.g. tinyint, int, smallint.

3.) should I use mix of datatypes I mean tinyint/smallint for category, int for Post, bigint for comments or the same datatype