What size softboxes should I buy for full body portraits?

Any size - depends on the tips of light/look you want.

Much of my full length uses a 140x100cm, but I’ll use larger and smaller boxes, beauty dishes and small reflectors too.

There’s no perfect/definitive answer ( like most things in life!) the single 35x160 is imho just a bit on the narrow side. You do have options with the ones you already have of ‘stacking’ them,  you don’t mention ceiling height which can be important, so either a 180x60cm or 120x90 configuration is possible and whilst you’ve used up the lights you have that combination together with a reflector ( preferably a V-flat ) would give you a lot of different configurations. A third light would give a bit more versatility but see what’s possible with your existing lighting.

Edited by Lightingman

I use two 60cm ones. If I want a longer light source then I can put one above the other. I also have a 90cm octabox which gives lovely light but because I have low ceilings I can't have directly above me to shoot.

For good effect you'll need more than one modifier, the only single full body light I've ever had any success using as a singular unit is a wheeled deep parabolic with 1000w head. Having sad that, it depends what you're shooting and attempting to achieve. For an easy remedy a beauty dish for upper and say large modifier covering remaining 2/3 of lower. Add accent lights /reflectors etc to compete desired look. I've pulled off full length with a couple of portable deep parabolics stacked upper with scrim and grid, lower providing specular/shadow for clothed part.

I used to use softboxes in a small area and the only type that would give me the control I needed were strip boxes with a very tight 30 degree grid. I had a large and small elinchrom with light tools grid.

The large one was the only one that could reach feet and head and the smaller one was used as rim or fill.

I tried all sorts of rectangular and octa boxes but even with flagging they spilled all over the place, which suits some but not me.

I personally have ditched softboxes as I favour really tight light sources with grids, reflectors or beauty dish which are great for my style of photography

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Edges above darkened in LR, by the way.

It really depends on what sort of light you want for your creative vision - bigger light sources closer to the subject will create softer light with more wrap around (depending on using a grid or not), smaller light sources further away will create harder light - the way to tell the difference is to look at the shadows, the harder the light the more sharp/defined the shadows.   If you want a more detailed explanation then I really recommend looking up Joel Grimes on YouTube and maybe buying his Dramatic Portrait course.

most of the studio shots in my portfolio were done with a single 120-150cm octa feathered love this type of light. done in a 6x3m room

I often (but not always) use a pair of these things...

A pair of 140 cm x 40 cm Dynasun soft-boxes on wall mounted extensible booms, typically powered by 100 joule to 200 joule studio flash. They have double removable white translucent baffles. These seem to deliver a reasonably good spread of light. They are good for lighting even very tall models (<1.90 m). Lass in the photo is 1.72m barefoot, wearing 10cm heels. They are also particularly useful when I need to light a white backdrop.

This shot is not a particularly accurate representation because she is leaning to the camera. It was taken to show off the studio, rather than the lights. And that bloody mannequin is in the way too! But hopefully you can get the idea.

Big soft-boxes are also a tad unwieldy. These are now almost ten years old and have also acquired a somewhat battered appearance. They are also a bit fiddly to build due to the springy steel rods that form their shape. Fortunately for me, they just squeeze in neatly on top of my Dexion racking, so I never actually have to dismantle them. I paid just under fifty quid for the pair, new, IIRC.

Edited by waist.it

For comparison. They are the small softboxes I mentioned, just very close to the camera.

Huw said

So, for a full length shot, bounce it off the wall, or off a reflector.

I was going to say something similar. Use a 6x6' bounce and a flash with a standard reflector + barn doors and you can have an absurd number of looks:

6x6' down to 6x6" if you want;

Spin the barn doors through 45 degrees, turn your 6x6' into an octa;

Crosslight the subject with a double black scrim, two lights for the price of one.

What Huw said: bounce the light.

As a very rough starting point… in a studio a 4 foot softbox will light 4-6 feet of someone from 4-6 feet away while still working as a soft box.

In a small space things get much more complicated: the way the light bounces around will totally depend on your space.