The use of "the" is fixed. ["the" + comparative expression], ["the" + comparative expression] It's a hard area of grammar to master so feel free to ask for more feedback. The more you ask, the better you will get!

" the " is definite article. when ''the'' is used with house we say about a particular house not any house .. which house?? -- bigger house. So when you say a house, it may be any type of house small,big,medium size.

Thank you wanda you answered my question

You definitely need it, because it sounds awful without. :) It might help you to understand if you expand the sentence: "The more children you have, the bigger will be the size of the house that you will need".

It's longman pbt example