dy leng das ist dastu hinter deinem swert stest und reckst dich / dy mass ist dastu nider stest … mach dich klain mit dem leib so pistu gross in swert
The LÄNGE is, that you stand behind your sword and stretch yourself / the MASSE is that you stand low … be small with your body so you are large in the sword.
“Maße, Maze, mediocritas, temperantia” is one of the 4 cardinal virtues (Plato, St. Thomas Aquinas – see also The virtues of fighting – The Liechtenauer manuscript GMN 3227a). So translating Maße with measure is a modern concept where measuring anything is common.
The term Misura is as far as I know not present in the 15th century. It appeares in Italian manuals of the late 16th centure. Fiore uses gioco/giochi but this describes an other concept than the later misura. Interesting to note, that one valid translation of misura is modesty, Mäßigkeit (comportamento).
It has a lot to do with the things you learn as a noble and educated man: the virtue ethics. Aristotle had no problem with fun as long as it is not to much. For Aristotle “Maße” was the perfect balance. Not too much, but enough. In medieval times this changed to mediocritas and many people associated it with “as small as needed” as you may find it in “mediocre”. “unmaze” (latin gluta), gluttony was one of the deadly sins (mostly associated with eating habits). Committing a deadly sin was feared a lot, so that the other extreme was sought to be rewarded in heaven. Not only in the works of Thomas of Aquin you find the idea to fight desire to prevent “unmaze”. Making the options for sinning so small that it is nonexsitent, was creating the “chastity”, that became a knightly virtue.
And that is what you find in the saying quoted in the Wallerstein: be small (modest) with your body so you are great. As the body is the tool for committing sins that spoil your soul, so greatness comes from the one who makes his bodily desires small.
Of course the sentences have very practical meanings in a fencing book. If you stand low your ability to absorb and generate power is greater – so you are greater. You can even use thrusts to block strikes. If you stand high and stretch yourself you have to hide yourself behind your blade because you have less power in your stand. You need strong strikes and cuts to defend yourself using the force of the accelerated blade. You must always stand behind your blade. So with the last sentence the author is recommending a lower stance preferring a more defensive way of fighting.
The correspondent piece of fighting art on the very same page gives the sentences a slightly different meaning to “Länge” (length).
…so du mit ainem vichst und zu ihm kumst an das swert das ir paid an hapt gebunden so reck dein arm und dein swert lanck von dir un secz dich mit dem leib nider in dy wag und sich dastu leng und mass in dem swert habst so magstu arbaiten und weren alles das dain notturft ist…
…as you fence with someone and you come to him on the sword, so that you both are binding [in the blades], than extend your arm and your sword long from yourself, get low into the scales [sit low in balance]. And see that you have length and smallness in the sword, that you may work and defend all to your needs…
In the fighting example, the principle of “long arms and short body” is explained, that is very feasible to weapons that are smaller than an arms length. This is irritating because a two handed longsword is very much capable of hitting someone standing very low with extended arms without endangering oneself. So this is to be recognized as the first beginners lesson, where the protection of the body is the elemental thing.