UK nicotine e-liquid comes in 10ml bottles at 20mg/ml or below, so a 10ml bottle is the standard size for starters. Buy one or two to test before you stock up, match the liquid to your kit (nic salt for pod kits, freebase for bigger tanks) and check the label for strength, ingredients and a best-before date. Buy from a retailer that checks you're 18 or over. Want to buy 10ml vape juice...