Before you build a scale model, you need to check its actual size. While a concept sketch can give you a hint as to proportions, it won’t tell you if a handle is the right size for a hand, an opening easily accessible, or a base the correct size for a particular object. A full size check on paper might reveal these issues with a minimum of effort before you start cutting into cardboard or foam.
Pick one key view of the object (usually front or side view) and draw its outline at full size with a ruler and/or measuring tape, not with an eyeballed estimate. Place known objects around it. A mug, phone, book, drawer or even an existing handle will give you a point of comparison when judging width, height, and reach against the everyday object.
Cut out the outline and put it in its intended working context: hold it against a shelf, desk, wall, or worktop. For a hand-held concept, place your fingers in the grip position. For a vessel, mark the opening and see if your hand comfortably enters. For something with buttons or moving parts, check whether key contact areas are seen and accessible.
Scale problems might be hidden since perspective drawings make big things look small, and small details look big. A handle that seems generous when viewed on paper may be too small once you have full size fingers on it. A base that seems sturdy on paper may be narrow in proportion to the object height at full scale. Likewise, you may run into issues with the volume of the whole thing if it clashes with something nearby, even when its silhouette looks fine on its own.
It’s worth doing a full size check with two or three variations, rather than just one, and correcting just that single drawing. Each time, vary one of the dimensions such as width or handle thickness, height of the main body, diameter of an opening, or depth of the base. Then compare the three versions side by side to get a better sense of them working in the same scenario. This makes scale a design choice rather than a shot in the dark.
A paper outline can’t be a replacement for a physical mock-up which can test weight, wall thickness, texture, joinery, and overall three-dimensional balance. But it can save you from making a model that you build around a set of proportions you aren’t sure are right in the first place. Before cutting cardboard or foam ask yourself: Does the object fit in its setting? Will your hand have the right amount of clearance? Are its key sizes in proportion to the job it is to do?