[A.J.Armitage]

They wouldn't need a man-sized piece of glass. They'd only need to place several end to end.

Wouldn't the place where the pieces met leave some kind of imprint on the pattern?

Or would the waves or lumps in the glass?

This seems like something to check.

[Swordmaker]

Wouldn't the place where the pieces met leave some kind of imprint on the pattern? Or would the waves or lumps in the glass? This seems like something to check.

Checking is unnecessary... the Shroud's image has been published and examined minutely... no artifacting of overlapping or butted together glass panes has been observed.

[Salamander]

Even perfectly cut and squared pieces of glass would exhibit refraction where the pieces butted together, thereby altering the light rays of the illumination source.
I would guess that the refractions would somehow, if only subtly, leave their distinctive marks on the finished image.
If a camera lens or darkroom enlarger has the slightest scratch or imperfection, that scratch or imperfection will result in imperfect light transmission and -will- show on both the negative and positive.
Let them duplicate their results with panes of glass of a size and quality that would have been available in -that- time period.