My dad own an old 86 Hilux LN65L-MR. It is a single cab with a long bed and for decades the floor's insulation have been missing. I do not have access to the car right now to measure the cabin ourselves.

Can anybody tell me the area the whole cabin floor so I could order the insulation material?

 

Can;t help on size, but will share what I used on my 86 X-cab interior engine bulkhead, kick panels, floor pan, transmission tunnel, back wall, and under the door card for insulation and sound damping. Called Frost King close cell foam (1/8" thick), self-adhesive backed, foil top layer in 12" x 15 foot rolls. Now $24 a roll. It like a poor man's Dynamat. If you want more deadening I peeled off the foil layer and added a 2nd layer with the foil in place on the tranmission tunnel. A lot of sound emanates from there and the shift level hole which is way bigger than needed.

I used the better metal foil tape to seal the seams. Not needed but I did it. The cheap stuff doesn;t stick well to the insulation foil. Get the good stuff.
Easy to apply as you pressed it to fit the shape of the panels.
It easily fits under the OEM insulation backed carpeting.
It's been 7 years and still strongly attached.
It does make the truck much quieter. Even the mechanic who replaces the rear axle bearings and took it ion a test drive couldn't believe how quiet it was.

Just my experience as in my experience.