You can proof it all in one big container if you want, there's no reason not to and it's done all the time in the industry. I doubt a big saucepan is going to do it for you, this dough is going to rise at least 3 times its size, maybe more. A very large stock pot may work. The other option is no container at all, if you have a large flat surface you can rise it as a big blob, just be sure to oil it really well first and cover the entire dough to keep it from forming a skin.
The real consideration is how you're going to make that much dough in the first place as 5kg is an awful lot. If you are going to do it all at once you'll either need access to a commercial mixer and dough hook or have very strong arms. More likely you will need to make it in batches to keep it manageable in which case you will want to rise them separately, so you can monitor their progress individually. If you make them separately and put them together for their first rise you will get a very inconsistent product as some will be more proofed than others.
You can make it in batches ahead of time, once a batch has tripled in size stick it in the fridge to slow it down, you can then leave it until you need it. Make it into balls and let them do their second rise while they get back up to room temperature. Note with pizza dough a second rise isn't necessary, you are going to knock all the air out anyway when you roll and stretch it.