If there must be room for mounted combat, we have:
Field of the Cloth of Gold: 150 paces long (375 feet assuming a pace of 2.5 feet)
The field for Lord Scales vs. the Bastard of Burgundy, 1467: 120 yards, 10 feet by 80 yards, 20 feet. (370 feet by 260 feet)
The Passo Honoroso, 1434: 146 paces long (365 feet)
Lists at the Castle of Rosenburg: 153 paces by 60 paces (382 feet by 150)
Lists for jousting at the priory of Saint-Martin in Paris: 96 paces by 24 (240 feet by 60)
The last is clearly an outlier, and was apparently used for many years for judicial duels, being converted to jousting lists as judicial duels became rare. Judicial duels were often fought on foot.
If only foot combat was expected, the field could be smaller. A field 21 yards square was prepared for a judicial duel in Tothill Field in 1571.
The Ordinance and Form of Fighting Within Lists written by Thomas, Duke of Gloucester before 1397 specifies a field of 60 by 40 paces. While not explicitly for foot combat, the only weapons mentioned are long sword, sword and dagger: the lance is conspicuous by its absence.
The regulations of Phillipe le Bel for the judicial duel, 1306, specify a field 80 feet by 40 feet.
The combat by consent between Galiot de Baltasin and Phillipe de Ternant in 1446 gives another clue to the size of their lists. When the met with lances, after each encounter, each was allowed to step back seven paces before coming again to meet their opponent. This suggests that those lists were no smaller than about 20 by 30 feet at a minimum.
The 15th century Linatges de Catalunya directs:
The lists for foot combat should be 40 paces square; those for combat on horseback, 80; those for a tourney of four against four should measure 120 paces; those eight against eight or ten against ten, 150.
While these lists may seem excessively large for single combat on foot to those interested in the modern recreation of those combats, it’s important to remember that it was typical for each champion to have their own arming pavilion within the lists.
Fallows, Noel. 2010. Jousting in Medieval and Renaissance Iberia. Woodbridge, Suffolk: Boydell Press. p. 193