The paper size decides the form design of the invoice PDF. Zoho Invoice support two paper sizes. The default paper size is A4 (210 × 297 mm) - the most frequently used paper size. The other available option is letter (8½ in × 11).
You could select the preferred paper size by customizing the invoice templates. All you need to do is to get into the ;
Settings >> Preferences >> Invoice Templates >> New Template >> Paper Size (A4/Letter)
Zoho Invoice FAQ