We don't have to fear much from spiders in Belgium. Lots of them, but mostly small and their venom is mild.
I'm no specialist, but I think that one of our 'nastier' species is this one (called épeire diadème):
You'll see those very often, the adults are usually 2 cm diameter(with legs), I've already seen some individuals around 3.5 cm.
Being bitten by an adult is far from being life threatening but can lead to fever, swollen limb and great general discomfort for 2-3 days.
Our biggest spiders are around 5 cm diameter but most bites aren't worse than a wasp sting. And if I'm right there are no allergy issues with spider venom.
A couple of years ago I was really afraid of spiders. Then I bought a house which turned out to be 'the Spider's Palace'! Dozens of fresh webs in the basement and we regularly see those critters running around in the rooms. Well it did a lot to calm my fear. I won't leave one running on my bare arm but I don't flail hysterically anymore when that happens... I rarely kills spiders now, I try to put them outside instead. And I must say that their web makes useful mosquito netting in the summer, as we leave near a big pond...