Reply To: Different text box sizes on master pages

Have you set up the document using Facing Pages and Master Text Frames?
With your B-Master after you have created it open it, highlight both pages in the spread, then go to Layout – Margins and Columns and adjust the bottom margin and the master text frame and the guides will change on your master and all pages you apply it to. You can then also add your page number box at the bottom.

https://helpx.adobe.com/indesign/using/master-pages.html
https://helpx.adobe.com/indesign/how-to/work-with-multi-page-documents.html