Even a handle that appears spacious on paper can actually hinder the way a hand grasps it. A hand needs space to get inside. A palm needs space to make contact with the shape, and a wrist must not be forced to bend. These considerations should be taken more seriously if the object you are using is heavy, if the handle must be used often, if it must be used for a prolonged period, or if the hand may be wet or gloved.
Start by considering how the handle will be used. Lifting a box, pulling a drawer, pouring from a can, or twisting a bottle lid all call for different things from the handle. A carry handle might sustain a load for a while, whereas a handle for a drawer simply transmits a brief pulling force. The expected load, the direction of movement, and the position of the object being used can influence the dimensions and location of the handle.
Space for the finger to enter and move around is clearance. A very small opening for the finger may allow the fingertips to engage, but may not be wide enough to form a secure grip. A deep space may provide enough room to move the hand, while at the same time sticking out a long way from the surface. Test the clearance against a real hand instead of looking at the drawing. Keep in mind that different people are going to get a different view of the handle.
Create a quick full-size model for the shape of the handle out of folded paper, a piece of foam board or several layers of corrugated cardboard. Fix it temporarily to a box or object that has a similar scale. Move slowly as you test it. Get close to the handle. Reach with your hand, move your fingers toward the handle, grasp the handle, lift or pull, and let go. Watch where your knuckles, fingers, the palm, and the wrist make contact.
After your first test, change only one aspect of the design at once. Increase the space that the fingers need in order to enter, increase the surface to make contact with, soften one edge, or tilt the handle. Repeating the same motion after each iteration makes it easier to compare the results. Changing a number of parameters at once might lead to a better result, but you might not know which parameter you changed led to that result.
Positioning can influence how the handle is positioned or how easy it is to reach. A carry handle that is placed off-center can make the body lean when it is lifted. A knob that is placed too near an edge can make one side of the top lift higher when it is lifted. For a bigger object, the handle is visually easy to locate but hard to reach from the direction of the approach. Taking pictures while testing the model makes it easier to see how the wrists bend and how the body reacts during movement.
Your final test does not require a finished handle that has been finished in every detail. It just needs to show that the space required for a good grip, the space required to let the hand move, the angle, and location all make sense for the intended activity. During your last test, check that the hand has relaxed fingers, a straight wrist, good balance, and that the hand can be removed easily without scraping the knuckles on the edge of the handle.