Size? I dunno. Should steel be avoided? Probably not, as I'm pretty sure all screwdrivers are steel (says the guy who makes titanium screwdrivers in his spare time).
I think you'd be best off grinding a screwdriver for this, if you're concerned. 0,6x4,5mm seems to be a good starting point, but you'd need a couple licks on a stone to get the thickness right (and cut the width down a bit while you're at it). And start with a screwdriver with a decent sized handle too, I find that it makes a huge difference being able to push down and put some torque into it. Much easier to mess up a screw with a small screwdriver than a big one...