All of the
dresses do not need to be the same style. If you choose to go
this route, all of the dresses should be the same color (and possibly
the same fabric). The hem lengths should also be the same. Another
option is to have the dress custom made. Another is to order from
a company that carries large sizes as well as regular sizes. In
this instance, you may be able to find flattering dresses in the
same fabric and color without the cost of custom made dresses.
And, finally, you can let each bridesmaid choose her own dress.
Give them guidelines on style, color, and hem lengths. The fabric
will not be the same.