Hexi. is dangerous if used in a confined space with no ventilation. (likes of snow holes, deep caves, etc.), But only if used a lot, might cause a head ache, feeling sick, if used in less quantity with out proper ventilation.
For the small amount you'd need to light a stove, I don't think would be a problem.
I don't think you would need to use more than the size of something like an Asprin tablet to light a stove. Would burn for around 2/3 min. Maybe more, a full block of Hexi. normally lasts about 20 ish min.
Normally enough to heat up a litre of water, (depending on how cold it is), or heat a rat. pack.