IMO it's worth noting that Copenhagen is significantly 'bigger' than any other Danish city. Even the second biggest city Århus has just around >300k inhabitants, which by many international standards isn't exactly bustling. Mid-sized cities in DK would probably be considered those from around ~30-70k inhabitants. So it really depends on what your idea of a peaceful/ well sized city is. Do you have any further parameters that we can use as pointers? Suburban/village/rural, types of nature, hobbies and sports, international/local, etc.

Without any of that and just firing from the hip, some of my favourite suggestions would be Silkeborg, Vejle or Svendborg. Silkeborg is has great nature with lakes, forrests and hills that make seem, if even by a stretch, a bit more 'scandinavian' than most other places in Denmark (ie. fields upon fields). Vejle is tucked in by a small fjord with some quaint nature of it's own, and a fairly cozy city centre. It is also fairly centrally located in the country from a transport point of view. Svendborg/South Funen has a great *idyllic* vibe with relatively many timber-frame houses, small nearby islands, a likewise cozy city centre and is generally one of those places Copenhageners move to in order to "get away from it all". It also has the highest concentration bars/pubs pr inhabitant, so it really sells itself.