is there a typical size for heater shields?
(lets say a knight who fights with a shield and arming sword)

from an article here on myArmoury i read that heater shields was most popular from late 1200s to early 1400s.
so that should give an approximate time period.
i have also read that plate armoured knights discarded shield for two handed weapon later in history, when, i do not know.
but hopefully it won't coincide with heater shield and arming sword fighting.

I don't know if there was a typical size historically, but as a modern re-enactor, myself and many of my friends have found that building shields to an individual's measurements will give you great results. This way the shield is the minimum size (and more importantly: weight) to protect the fighter. For heaters: the height of the shield is determined by the vertical measurement between the top of your inseam and the bottom of your chin. The width of the shield is determined by the horizontal measurement across your shoulders. (shoulder tip to shoulder tip)

Based on period art, I'd say footmen's heater shields were a bit larger than that - at least the earlier ones in 13th century. They seem to be roughly half of warrior's height, which is 80-90cm / 31-35" on an average guy.

thank you for your replies so far.

but can somebody please explain the word 'inseam' to me, it didn't work in google translate.
and its kind of essential to understand the first reply.

13th Century- 76x56-94x60cm
1st half 14th Century- 54x40-80x75cm
2st half 14th Century- 49x36-73x60cm (memorial?)

Smallest and largest shields that I know.

The size of heater shields vary with time and use. As a very rough guidline, they become smaller as time progresses. A 1100's heater might be as long as a kite shield, a 1300s one only 50-60 cm. A typical 1200's infantry heater would cover you from shoulder to knee (about 90 cm).

Shields stop to be standard equipment sometime around 1350, though they are still around.

Practially, plywood sheets come in 244x122cm, so your shield will most likely be 122-90x61cm.

A related question: what is the difference in performance between a heater shield and , say, a circular shield? Both were used from horseback. Why would the heater typology be a better shape?

so around 50x75cm (20x28"), a bit larger than i had envisioned.
thanks for your replies, you have been most helpful.

Dan Howard asks a interesting question, i would also be interested in the answers.
would the size of a round shield also correspond to body size?

Steffen; What are you going to use it for? Display? living history? combat?

Dan: <Rant>The development from the round, domed shields used by frankish or eastern cav to heater shield goes by way of the kite.
though my experience with domed shields is limited(mostly based on one of our guys having a large specimen) , they share the down sides with their flat bretheren; for one, they are as broad as they are tall. Thus a round shield that covers you from knee to shoulder (about 90 cm/3ft) will also be 90 cm broad. Such a shield would not be practical for horseback use.
The result of this is eastern cavalry using smaller, 60 (2ft) cm round shield, that can be moved over the top of the horse. This does however not cover the legs, and the shields are to small for massed infantry combat.

there are several ways to envision the creation of the kite. One is to imagine a small round shield stretched down to cover the leg. The result is a shield that perfectly fits the silhouette of a standing man. This means less "excess" wood to carry, the hand straps gives better control as opposed to the central grip, and you can pack you infantry as tightly as in a flat roundshield shieldwall.
At the same time, the early kite has a vertical grip that lets it be tilted over the back of the horse to cover the chest when fighting enemies on the right side, allowing its efficient use on horseback.

the heater is a result of two upgrades to the kite, namely the addition of square corners and a widened base. Most likely, these where developed to give better protection against angled attacks (weaknesses I have noticed when switching between kite and heater shields.) As leg armour for knights developed further, cavalry heaters shrunk, until they where back to the 60cm pass-over-the-horse dimensions. </rant>

The difference is mainly better protection for the shoulders, and coverage for the tights on early models.