If you wait too long and they are much bigger than that, prune them back as you plant them out. It helps reduce moisture loss while the root system settles in. Make sure that whatever support you have in mind is strong enough for the vine.
They do get huge and heavy pretty quickly and need something sturdy. Also be aware that a vigorously growing passionfruit will climb over any- and everything it can reach and can quickly smother plants. Make your own life easier by growing passionfruit away from other shrubs and trees. (Note to self: I should heed that advice myself.) Be careful to disturb the roots as little as possible. Dig a big enough hole, at least twice as big as the root ball, and ideally mix the soil with compost before you back fill. In the early days you may have to train your vine up the support by carefully tying it. As soon as there is something for the little tendrils to grab hold of, say the first wire on your trellis, it will climb on its own. Passionfruit needs a steady supply of both water and nutrients. Make sure you don't have water puddling or not draining away. Too much nitrogen (most commercial fertilisers are heavy on nitrogen) will lead to lots of soft green leaves, attractive to all sorts of insects and diseases, but you get little fruit.
So, lots of compost, lots of mulch, and the odd sprinkle of a balanced, organic, slow release fertiliser. Now here is an issue that I only became aware of when readers started asking me about it. Or rather, asking why their healthy and profusely flowering passionfruit vines did not set any fruit. Or why the fruit was small and hollow rather than filled with delicious, juicy pulp. There are several possible reasons for this: Even though the purple passionfruit is self-fruitful (meaning a flower can be pollinated with its own pollen) wind pollination does not work with passionfruit. Passionfruit flowers rely exclusively on bees for pollination. You can help your passionfruit by planting plenty of herbs and flowers in your garden that attract bees. While purple passionfruit is self-fruitful, the yellow passionfruit is mostly self-incompatible and needs to be cross pollinated with another cultivar. So if yours is only the passionfruit vine near and far, you need to plant a second vine of a different cultivar. Pollination works best in warm temperatures and humid conditions. If the air is too dry, or it is too hot or too cold, and also during torrential rains, pollination will be ineffective. Since I grow several varieties of passionfruit in my garden, I never had a problem with pollination. Sure, I did notice that there were times when I had no fruit despite having many flowers. I contributed it (correctly) to the weather/temperatures and didn't worry about it, since I knew that sooner or later I would be showered with passionfruit. The tropical varieties fruit quicker than the purple passionfruit. Any passionfruit will reach maturity sooner if growing in a warmer climate. A passionfruit vine planted in spring fruits sooner than a passionfruit planted in autumn. In ideal conditions (early spring planting in the tropics) you can get fruit within six months. Autumn planting in a cooler climate means you may have to wait for over 12 months. That's what I love best about growing passionfruit. Dropping on the ground does not hurt them the least bit, they will neither rot nor will they get eaten by insects, birds or anything else. (Ok, possibly the neighbour's kids.) Just collect your passionfruit as often as you feel like it. When one of my passionfruit vines is in peak production I usually do it twice a day. At other times I will pick up my red passionfruit daily. It has a much thinner skin and in my warm and humid climate I find it goes moldy on the inside unless I put it in the fridge straight away.
My yellow passionfruits do not mind lying on the ground in the sun for a few days, and then sitting on the kitchen bench for weeks. They shrivel up and look awful, but they are still juicy and delicious inside.
Some of my passionfruit vines raced up into some huge trees and I can't get to them. Anyway, most people grow passionfruit on much smaller structures, like fences or trellises. If you don't prune your passionfruit you end up up with a thick, tangled mess of dead wood, and a plant that has lots of problems with fungal diseases.