Sit down or keep it informal?

With that amount of people I have to agree with your brother. The problem you have is when you have different sized groups so you might have a family of eg 4 and one 3 choose to sit together on a table for 8 and then there's lots of random odd seats. Not a problem if you have lots of seating obviously.

We are having a similar thing, 150 people with a hog roast. We don't want set tables and are basically having a 'find seat, plonk bum' policy. I think that when you're ready to have food a small announcement by an by an usher should get everyone heading for a chair and shouldn't have any problems. I've been to a wedding like this before and everyone found a seat with people they wanted to sit with and were perfectly able to organise themselves. Especially if you let them know before hand that there is no seating plan, just so there is no confusion.

Another alternative might be to have the speeches at a different point of the day if you are worried about this. One wedding I went to had the speeches when everyone was stood around the dance floor, just before the first dance.

I agree with PHB. The other thing I would check is how quickly the hog people can serve that many Guests. I went to a wedding that also did this and it took a long time. People were made to queue and it wasn't enjoyable for them. When a meal is being served you'll get about 7-8 waitresses doing it so the turn around is quicker. For a hog roast there is usually the person carving and the person serving. It's not so bad for the evening because people would have had their wedding breakfast already so wouldn't be as hungry. Just wanted to give you a heads up so you can check x

If you leave people to sort themselves out you'll find odd numbers on tables, some people might find themselves without a seat or split up from their friends. Why not tell people which table to sit on but leave them to choose their seat. We did that and we had a BBQ.

We debated this one too..... Having about 90 including 18 under 6's for the hog then just a couple extra for the evening buffet. We were just going to let folk sit at whichever table they fancied as we want an informal do. However, we have now chosen to do designated places so we can ensure everyone is seated for welcome drinks and informal speeches, then we can send those with kids off to be fed first, then others to follow. If they are all seated to eat, we canensure everyone is fed. They can do whatever they like for the evening tho xx