I would like to copy my Ikea bed into Tas Oak one day.
From the beds that I have built, I measure the mattress and work from there. That will give you your interior dimensions of the frame. Rockler sells non-mortise bed rail hardware that makes things easier too.
Yeah, Justin's method is what I use, too.
Always start with the mattress size and make sure you design in room to actually be able to get the sheets on the bed once it is built.
If you are using a flat surface for the mattress then this isn't such a big issue. But I once built a bed where the mattress sits a few inches down within a frame and I got lucky that I made the frame a few inches larger than the mattress dimensions. If it's too tight a fit somebody will curse you ever time they have to make the bed.
As for plans, there's quite a few websites in addition to Build Something. Please see the below discussion thread. HTH!