I'd wait - hard this time of year - until you've seen how they react when the mare is in season.........she'll behave differently then and there could be injuries
tbh I'd keep them separate and alternate who has the big field and who has the little field.
No way, I wouldn't put any horses into a field together straight away. You should divide the paddock for a least a few days to allow them to settle. Putting strange horses together is a recipe for disaster.
My mare was kicked on her hock last year when a new horse was put straight in her field. I nearly lost her as it was such a bad injury.
Accidents happen but best to do all you can to avoid them and introduce new horses gradually.
I'd divide the big field in half and put one one side and one the other...just until they get used to each other, gradually introducing them to each other.
Of course this time of year is different and you may well have to split them up again when she's in season...and do the same again!
Whichever field you use, don't think it really matters although I am always partial to using the largest field for situations like this, the biggest thing to consider is do they both know this field WELL? If they know where all the twists and turns in the fenceline are then they should be able to get themselves out of trouble.
I would use the big field, less chance of one pinning another in a corner (hopefully!). With new horses I usually let them speak over the fence for a few weeks, then let the new one into the field, but only if they're settled over the fence and not trying to kill each other. Saying that, when I moved Chex he was shoved stright into the field with a mare, but she was 27 and not really up for fighting, and neither was he.
My gelding is now out with my friends mare and gelding, we put them straight into a large field with no issues, they have been together now for three weeks and still no issues.
I think it all depends on how personally you feel comfy i just prefer larger field as they can get away should anything happen imo
I haven't read other replies on purpose, as for us we put the new horse in the field, or one of the old horses and add another horse time by time, waiting a day or two between each horse.
Therefore I would let them out in a big field, def. not the small paddock as risk of injury, although an arena might be ok.
A big field they should be fine. Have a bucket of carrots or something on stand by in case they start chasing each other (that's if they'll pay attention..)
We've only had one problem ever, and this was the pony chased the horse, we took pony out, replaced him the next day and not a problem since!
we dont get an option where we are other than to keep one or two the bosses in for the first day or two, other than that geldings and mares are split and we have to just put them out straight away and cross fingers.
Luckily they are all nice chaps so nothing nasty ever happened with any of the new boys.