(I will answer this question for the enclosure (unsupervised) only. For the daily garden movement you need the "rabbits stay in" rules and can read the question Should I be worried about neighbors Cat in Relation to my outdoor Rabbits? )
First you should know which rabbit specific predators are common in your area. For western Europe this are fox and marten.
So your boundaries have to hold fox, marten AND rabbit in place (two at the outside, one at the inside).
[Edit: What to do, if you have raccoons in the neighborhood, you can read here: Racoons: What sort of protection do my rabbit enclosure need? ]
It should be point welded. This means, every cross of two wires should be welded, instead of woven wires. At woven wires it needs only one cut and the almost whole grid could rip, so rabbits or predators can force themselves trough.
If you use thin wire (more than 1 mm/0.04 inch), the distance should not be larger than 19x19 mm (0.75x0.75 inch), so the rabbits can not use their teeth on it. Bigger distances have to be from thick wire (which the rabbits can not bite in pieces).
Marten is the smaller predator. It can use gaps of 4cm/1.57inch, so all distances should be smaller. Marten can use big forces for example to bent wire, so the bigger gaps should be very solid surrounded. (For example some produced rabbits enclosures use grid with 3x5cm rectangles. This could be bent by marten, so he fits through the resulting 4x4cm gap.)
There is a saying here: where a chicks egg come trough, the marten comes through too. (In tales marten is the chicks killer).
The wire should be processed, for example galvanized, to withstand the weather (rain -> rust).
The grid has to be properly attached. Attach it with full rectangles, not with only out sticking wire-ends. Make a pull test.
optional: Curtain against insects
If you have sorrow, your rabbits could get maggots or myxomatose, you can use this special curtains on the wire.
Requirement: no rabbit digging its way out, no predator digging its way in.
If the enclosure foots on solid, it is useful to sink stone plates ore grid between 30 and 100cm deep in the solid (12 - 40 inch).
A other possibility is to lay a 30cm wide stone path on the inside of the fence around the enclosure.
On both strategies one have to overlook the digging effort of the rabbits, but because the ways to dig are long, the rabbits can not dig escapements without one notice.
More building effort cause the next two suggestions:
One could remove the complete basic solid of the enclosure, lay grid into the ground and fulfill the hole. Or one could lay stone plates on the ground all in the enclosure.
Marten is a very good climber, so it is useful to close the upside of the enclosure (especially if they are small) with grid too (see point grid). Web is not suiting here, exception is wire-web, but this is very expensive.
If the fence of the enclosure is lager than 2m/79inch, one can add a second part above. This should be tilted about 30° to the outside. One could find such things for cat-security (they want hold cats in) but uses it opposite (to hold marten out). This 30° part should be very smooth (give no hold for predator) or with a electrical animal fence (like the one for horses or cows).