First of all if you choose a small mesh size there is no chance your rainbow lorikeets hurt themselves by getting caught in the mesh - for example poking their heads through. More importantly if you (like I do) build an outdoor aviary you will attract a number of unwanted visitors from mice to possums to snakes. By choosing the smallest mesh size you can keep most of them out - only the ants still make it into the rainbows aviary. Small size mesh may be more expensive but brings many advantages - including being able to use your aviary for any type bird in the future not just your rainbow lorikeets. Don't forget to buy high quality mesh which does not flake or metal poisoning can become an issue for your lorikeets.