Tip size on a plain slot driver is the width and thickness of the blade together, and both have to match the screw. A tip too narrow bears on the middle of the slot and cams out; one too thick will not enter at all and one too thin twists in the slot and rounds it. A rounded slot then has to be gripped from outside rather than driven from within it.
That is why plain slot sizes are quoted as a dimension rather than a number, at 3.5mm, 4.0mm, 5.5mm and 6.5mm here. Cross slot drivers are numbered instead because the tip is a shape rather than a width, and the number identifies which shape. Cross slot numbers run from No.1 upward, and the number is read off the screw rather than guessed.
Blade length is chosen against the depth of the screw rather than the leverage. A short blade keeps the hand close and controls the driver; a long blade reaches a screw set below a surface where a short one would need the surrounding material removed. Blade length is stated per driver alongside the tip size rather than being a range.
Slimline blades are the answer to a screw sunk in a counterbore, where a standard blade is too wide at the shank to enter even though the tip fits. Blades run 80mm, 100mm, 125mm and 150mm. Send the screw to 07754288583 on WhatsApp. Interchangeable blade drivers cover several tips from one handle on the same leaf.