A sensible rule of thumb is that standard size well built trailer will be 1000kgs - you should check and the trailer will have a plate on it to confirm.
Horses are very variable my 15.1 cob type was on the weighbridge at 580 kgs ! A friend's 17.1 hunter type 670kgs
Either of those would give you scope for tack, water, other normally horsey junk and not be right on the limit (which you don't want to be if you have to go up a hill or overtake something).