The print is 18" x 24" in US, Canada & Mexico and in Australia and New Zealand you have a choice of 18"x 24"/ 45.7cm x 61cm or 50x70cm. It's 50 cm x 70 cm for Europe and the rest of the world. We use museum grade, art matte, acid free paper and print using archival ink.
In the US, Canada and Mexico the print is designed to fit any 18x24 inch (45.7cm x 61cm) frame - if you want the mat surround then go for the 22x28 inch/60x80 cm frame with 2 inch/5cm mat.
For Australia and New Zealand you will need a frame that fits the size you choose 18 x 24/ 45.7 cm x 61 cm, If you want a mat surround then you will need 24x28 inch/60x80 cm with 2 inch/5 cm mat . For the 50x70 cm size for larger frame with mat you will need 60x80 cm with 5 cm mat.
https://www.framesnow.com.au/the-night-sky-frames/ specialise in frames for Nightsky prints for both 18x24 inch and 50x70 cm. sizes.
Amazon have endless choice and a lot of people recommend Target and Meyers. https://www.frameshop.com.au/photo-frames/ or https://www.framesnow.com.au/product-category/frames/?filter_size-cm=61-x-45-7-cm&query_type_size-cm=or If you're looking for something special then your local framing store should be able to help.
In the UK and Europe you can go for standard 50 x70 cm frame or the larger frame 60 x 80 with 5 cm mat.
Related: What is the Quality of the Print?
Hope this helps!