may or may not be relevant: a few years back when i got my first siqq fixxie it was way too small. i’d been riding a way too small roadie prior and that was fine but the added muscle strain of stopping with my legs worked its’ way up into my back and fucked my spine for a little while there. so yeah, don’t ride a frame that’s 6cm too small.
moral of the story: get a bike that fits right. (maybe you alraedy have that though…)