Hello,
I've noticed that the effect  of setting frame size is not consistent among the various methods of performing the task. I can illustrate this with the following example involving several steps:

1. Define the function

(defun testfn () "tests frame creation"
  (interactive nil)

  (other-frame 1)

  (let ((frms (frame-list)))
    (set-frame-size (car frms) <x-size> <y-size>)   ; e.g. <x-size>=76 <y-size>=40
  )

  (setq default-frame-alist '((width . <x-size>) (height . <y-size>)))

)

2. Start off with one frame, and create a second frame;

3. Evaluation of testfn will set the size of the second frame; at the same time
   the default size for future frames will be set.

4. Create a third frame. It will be seen that the size of the third frame is    different from the size of the second frame that was set in testfn. In my opinion    the default-frame-alist parameters should have the same effect as the set-frame-size parameters, but apparently they do not.
Regards,
Bostjan Vilfan