Is there a print preview option in Manager?
The exact print layout of documents generated by Manager depends on your operating system, printer driver, paper size, and other attributes you set. If you really want to see what something will look like, create a PDF.
Please add Print Options for printing Invoices such as Paper Size, margins,scale, layout to customize the Invoice print.
Changing fonts to accommodate multiple text boxes in A4 Sized Paper as shown in the above image
PS: @Tut would be great if you could develop a theme as shown in the image. 
A4 or Letter paper size is picked up from your email settings. Other parameters depend on your theme. If you want changes, build a custom theme.