A base layer changes the fit of the clothes worn over it. A jacket or pair of trousers that feels comfortable on its own may pull, bunch or become awkward when another layer is added. Try the whole combination before relying on it for a long outing.
You do not necessarily need to buy everything in a larger size. First work out where the fit changes. The useful question is whether the layers sit comfortably together and allow the movements you plan to make.
Understand the layer's job
REI's base-layer guide explains that a moisture-managing base layer generally needs contact with the skin. That is different from assuming that tighter is always better or treating an ordinary base layer as medical compression clothing.
A base layer, an insulating middle layer and a weather-protective outer layer have different roles. You may not need all three for every activity. Choose the combination for the conditions and effort rather than putting on every item because it belongs to the same outfit.
Try the combination in normal positions
Put on the base layer and the clothing you expect to wear over it. Sit, stand, reach forward and take a comfortable step. Notice whether sleeves gather at the elbows, the waistband becomes restrictive or the outer garment pulls across the shoulders.
Columbia's fit guidance describes a close fit without restricting movement. Use that principle to assess the actual garment instead of treating a printed size as proof that the combination must work.
If the base layer rides up when you move, check its length and cut. If the outer layer binds, compare another cut or arrangement. The solution may be a different shape rather than increasing every size and creating excess fabric elsewhere.
Check cuffs, hems and fastenings
Small details can become noticeable after several layers are added. Try the cuffs with gloves if you plan to wear them, and check the trouser hem with the intended socks and shoes. Fasten the jacket normally rather than testing movement with it left open.
Keep the checks ordinary. You do not need extreme stretches to decide whether you can reach a pocket or sit comfortably. A few familiar movements can reveal more than standing still in front of a mirror.
If you are comparing two combinations, use the same few movements for both. That makes it easier to notice whether the difference comes from a particular layer or from testing the outfits in different ways. Pay attention to where the fabric gathers, not only whether the jacket can be zipped. A closed zip does not explain the whole fit.
Keep an adjustment option
Think about how you will remove or add a layer if the effort or conditions change. Make sure the garment can be handled without an elaborate rearrangement of everything you are wearing.
After trying the combination, remember what worked. You can repeat a useful outfit without conducting a fresh test every time, while still checking changes in weather or activity. The aim is clothing that moves with the day—not a particular number of layers or a size label you feel obliged to keep.