Multi coloured foil "HAPPY BIRTHDAY"
Letter height approx 14" when filled.
Letter span: currently strung across about 12 feet/3 meters.
For easy ref that makes each letter (and the space between) just a bit larger than a full A4 page.
These are available fully air-inflated only and any that are going down can be refilled by inserting a straw in the valve as shown.
I figure you might be able to flatten them and go again with helium if you really wanted to - note the valve is self sealing and not plugged, which may or may not work with helium.
Happy Birthday to whoever wants it 🎊😁.

🎊Inflatable Birthday Decoration (Great Missenden, HP16)