good luck on your first HAED project.
I started mine last year , mine called for 25 count even weave but i switched to 18 count and bought the magic guide aida to help with the counting and keeping me on track.I was lucky and already had my threads but look on ebay as they are normally pretty cheap .
If the pattern calls for full stitches that is what you should do .
With the size of the material you need a 3 to 4 inch border for framing the piece as well so maybe that why is sounds so huge.
I have a round lap frame and use that for my big projects and just use hair grips keep the excess fabric from getting in my way .
These are only what i use, sure some of the members will be able to help you and point you in the right direction and welcome to the forum