I can't even make my skin for a 1024x768 resolution. My height stops at 661 and won't continue.

Edit: Please don't tell me to make my skin 800 by 600 considering that it also is a 4:3 perspective, BYOND's icon map stretching fails to produce.
You should be able to have any size.
Have you tried going to properties/options window and changing Width/Height numbers?
What about dragging corner of the window?
Hm. The most I can get it to is 722 by dragging it.
The window is currently limited to the resolution of the machine you're creating it on. Can you not make your resolution any higher?

It would be nice to have some sort of scrollable "canvas" to work within; I'm not sure if it's ever been requested.

You can change the size by editing the interface file in a text editor, but I think it will just change back if you open the window in DM.

Your best bet may be to design your interface at 800x600, and set up all the anchors so it resizes properly to 1024x768, and just winset() it to the desired size at Login().
My resolution is 1366x768, that is the highest it will go

Well, if you hide the taskbar, you should be able to use all but the last ~4 pixels.
For some reason, my computer doesn't want me to expand this window past 722 pixels.
You're making a mistake by designing huge. You should design small, and make it stretch to any size. Few users will have 800x600 limitations anymore, but 1024x768 (with a chunk taken out for the taskbar) is hardly uncommon, so you should always plan for the smaller size since that also avoids any issues where the system gets confused trying to place elements in a way it can't. (For the most part I think such issues have been long quashed, but I wouldn't count on it. Designing small is safest.)