Large heatbeds are often made by using several smaller standard ones, like 300x300mm. They can also be staged during heatup if insufficient AC is available. You probably want to use 240V for heating though, then you could have plenty of power. The print sheet spans the individual heated plates. The Prusa XL works this way with 16 smaller plates on a 360x360 print sheet, and the Voron Phoenix which is 600x600 using four 300x300 plates. If you use 16 300x300's that would be close to four feet.
Rolohaun has a series of DIY printers that are popular among home builders. Many folks buy the cheap Creality or similar printers and upgrade the heck out of them, or convert them to Vorons. The RatRigs kits are not open, so not really quite the same as DIY. DIY 3D printing is a separate world from the Bambu space.