Hi. We will try to answer this as quickly and as good as possible. 

1. Changing the size of the logo:  Activate the developer mode and navigate to Settings --> Technical --> User Interface --> Views. Here search for the keyword: external_layout_header. Edit this file and find this line: <img t-if="company.logo" t-att-src="'data:image/png;base64,%s' % company.logo" style="max-height: 45px;"/> and change it to: <img t-if="company.logo" t-att-src="'data:image/png;base64,%s' % company.logo" style="max-height: 70px, max-width: 70px;"/> You can change the max-height-width as you wish.

2. It is the same layout : external_layout_header for the invoices and for the sale orders too. So it will be taken from there. And for the footer it goes the same, just search for the view name external_layout_footer and make the desired changes there. It will be reflected on all printed documents that use the footer view.