I’m not sure when Flickr started to generate more sizes for uploaded photos. If you upload a photo, it will be uploaded to Flickr, and will be processed to generate multiple image sizes. Here are the sizes for reference — based on a photo taken using iPhone 5:

• Square 75 (75 x 75)
• Square 150 (150 x 150)
• Thumbnail (100 x 75)
• Small 240 (240 x 180)
• Small 320 (320 x 240)
• Medium 500 (500 x 375)
• Medium 640 (640 x 480)
• Medium 800 (800 x 600)
• Large 1024 (1024 x 768)
• Large 1600 (1600 x 1200)
• Large 2048 (2048 x 1536)
• Original (3264 x 2448)