K.S.
Have you looked at GMD toddler sized prefolds? They're much bigger than premium prefolds and fit much longer. Flats can be folded to fit pretty much any size (though you will definitely need at least 2 to absorb the waste from a toddler that size - I just match the two squares and fold them up together). For bigger covers, Thirsties large sized covers are a bit bigger than the blueberry or thirsties duo - but they only come in velcro. Many WAHMs will make fleece covers in whatever size you like. There are also WAHMs who will make custom diapers or covers to your measurements. You'll have to look/ask around a bit for those, though. With some simple sewing, you could "upcycle" wool sweaters from good will into pants or soakers to match measurements. Fuzzibunz sized diapers can be found as big as XL which is at least a full size bigger than the OS can go to. You might also look into Happy Heiny's pocket trainers. They come in training pants sizes and can be snap or pull on/off, and have a pocket so you can customize the sizing. I haven't tried them but they sound like a possibility. You could also try Snap EZ for life. They make diapers for special needs individuals in sizes through adult. I'm not sure I've heard any good reviews for them, but they might be worth a try.
Good luck!