There are numerous different types of doves, and since the birds are mentioned only once in passing in The Silmarillion, it's difficult to be sure exactly which kind is intended. The reference in question appears in a description of the pearl Nimphelos, which was said to be a 'great as a dove's egg' (Quenta Silmarillion 10, Of the Sindar) which at least shows that the putative author of that passage was familiar with the bird. Doves' eggs vary in size, but they typically average about an inch (roughly 2.5cm) in diameter, so Nimphelos would have been approximately this size. They also tend to be pearly white in colour, which presumably suggested the connection with the famous pearl.