Errrr.... strong enough. Check out the supply of compression springs in Bunnings. They don't do both short and heavy gauge together unfortunately. I redid my 8" reflector springs a few days ago. Bought longer heavy gauge springs and cut them in half
Filed the cut end fairly flat. Seems to be working fine.
Springs too weak and the mirror will sag. Springs too strong and you'll be building up your muscles turning the collimation knobs - any maybe risking stripping a thread?
Test - Collimate with the tube horizontal. Move the tube to vertical and see that the collimation only moves fractionally, if at all. Mirror sag should be quite obvious. Some collimation movement might not be mirror sag, but a bit of focusser draw tube slop or tube flexing?
If you now have a 3-4cm gap, you don't have to fill it completely with spring. You could fit a solid spacer over the longer bolt leaving a 1.5-2cm gap in which to insert a spring.
EDIT: Of course, it also depends on the weight of your mirror. If you are talking about the 8" skywatcher, then that is not a big issue. I'll look up the dimensions of the ones I bought and post here.