I believe the answer is no, Beleriand was not as large as the other areas of Middle-earth. As @maksim shows in his answer, the illustration from Karen Wynn Fonstad's Atlas of Middle Earth, published in 1981, gives a pretty accurate relative sizing of the two areas. I believe the common approach these days is to combine the maps based on the two common items in the maps, Tol Fuin (the remains of Taur-nu-Fuin) and Tol Himling (the remains of the Hill of Himring) to establish a common scale. That results in an image that puts Beleriand at a smaller size than the currently accepted answer.
However, as @Edlothiad points out the difficulty in combining the maps is that one is a flat Earth and one is spherical, and there's really not much we can do about that. I created an iOS app that combined the two and wrapped them around a spherical world. This is the mercator representation of that.
Also, it's important to remember that areas to the north, south and east of the maps in The Lord of the Rings and The Silmarillion as shown in this illustration are non-canon.