You would measure the force on Io from Jupiter's center.
dasviper is correct about the relevence of size. Gravity is acceleration -- on Earth, everything is accelerated toward the center of the Earth at 9.8 meters per second, regardless of it's mass. (The greater the mass, the greater the force, but that force will have to act on proportionally greater mass).
As it happens, luckily, I was doing this somewhat recently and still have the figures.
Io's orbit: 422,000 km from Jupiter
Jupiter's diameter: 142,984 km (at equator)
So Io would be 279,000 kilometers from Jupiter's surface. Quite a ways.
We can also reason the (the Velocity of Io) squared would equal (the Gravitational Constant) * (the Mass of Jupiter) divided by (the radius from center to center).
Mass(io)*Vē = 6.67*10^-11*Mass(io)*Mass(jupiter) / Radius(jupiter-to-io)
Then vē/r which gives us Jupiter's gravity as being able to pull Io toward it at 0.7 meters per second per second. By comparison, the Earth can pull on our moon at 0.0027 meters per second per second.
Hopefully I didn't confuse anybody. Can anyone check this?
Image of Io