EW effects the chance to hit for beam weapons, and the maximum range that missile fire control can lock up it's target.  The actual formula is "(Defenders ECM - Attackers ECCM)x10+% modifier if the ECM is greater than the ECCM.  For beam weapons this then is subtracted from the chance to hit as modified by range, and the target ship's speed.  For missiles this is used as a multiplier on the range of the fire control to determine it's actual max engagement range.

Beam Weapons
ECM = 3, ECCM = 1, net difference is 2x10%=-20% modifier on all to hit chances.  If the base chance to hit is 20% or less then the adjusted chance is 0% and the weapons will not fire at all.

Missile weapons.  Same as above, if the max range on the fire control is 30 million km then 80% of that would be 24 million km.

EW has no effect on sensor detection ranges, nor does the size of the ship make a difference directly.  Small ships howeve are often faster, and therefore harder for a given beam fire control system to hit, and have smaller resolutions which tends to reduce the range of normal sized missile fire control systems.

Brian