The raspberry pi can attach to any TV or computer monitor that has a HDMI connection port. If your TV does not have a HDMI port, you can purchase an adapter that will convert HDMI to SCART (around £12) or HDMI to VGA for around £5.

There is no "best size" for a digital display screen and certainly no need to use expensive bespoke monitors where a TV or computer monitor will do just fine.

The HDMI will provide the best quality image whilst SCART or VGA will be a lower quality output. The choice is yours to make