Ever tried to sign your credit card receipt on a tablet? It takes a bit of getting used to, but since so many coffee shops etc... now use those apps, users are 'getting used to it'. However, what about smaller devices? Apps that let you sign on an iPhone? Are there any studies out there showing what the minimum space needed is? How do users handle signing something on a tiny screen where they can't rest their hand? Are there any comparisons between finger signing and pen signing apps?
Here's some info I've found, and further information I've derived.
Summary: There is no standard established, but a comparison of 4 apps with "signature" spaces (Square listed here in detail) has given an approximation of 93% as wide & 75% as tall as the area the user thinks is meant for their signature. This means up to 1300x480 and down to 880x320, using 1100x400 as midpoint standard based on iPhone5 sizes.
Note: This primarily concerns the "size" question, but touches on all questions asked.
This MIT Article approximates the size of a human fingertip between 45 – 57 pixels. This is reflected in the Best Practices for touchscreen buttons being (roughly) between 44 and 55 pixels in width. iPhone best practice is "around" 44px, whereas iPad is 55px in standard resolution. Similarly, the Android Best Practice states a range (due to different screen sizes), but a common size is 48x48 HDPI for standard icons, but up to 97x97 in some cases. This is worth keeping in mind when positioning/sizing the layout/interior containers for the design.
While in most cases, a digital signature can be considered a legally binding signature signature (aforementioned delivery companies), not all cases consider a digital signature binding.
I've been unable to find an exact guide to the sizes of signature areas, but it's a well-affirmed notion that bigger is better. Any app I've used, or app that anyone in the design community I've asked has used, has had a generally oversized area for capturing signatures.
An example is Square and their signature design. They have a huge signature area, along with a basic navigation, including a "clear" function.
Here is an article about Square's signature UX process. It's less about the exact size of the signature area, but more about how the appearance of the signature and the experience of drawing it out should be.
Just as a quick comparison, using 1136x640 (iPhone5, landscape) as a standard, Square's signature pad is roughly 1100x540 (rotated). The implied (n1) signature area, or "where a user is most suggested they should sign", is ~1100x400.
Scaling this approximation of 1100x400 to the variation in sizes between the iPhone and Android button sizes, up to 1300x480 and down to 880x320 would be feasible as the implied signature area.
The responsiveness and look of the signature is most important. If it looks blocky or unnatural, like every FedEx signature I've ever signed, it will not satisfy the end user. As far as the space itself goes, these approximations (explained a bit below) are 93% of the width, and 75% of the height of the area the user identifies that the signature goes into. There is not a standard, but this is generally large enough, and is an approximation of what many touch-signature apps have implemented.
Fluid gestures can help alleviate the awkwardness of not having something to rest their hand on. The less time they need to spend pressing against it, and the more responsive the signature drawing is, the better.
I considered the "expected" signature area and the implied area as starting at the signature line, not including (as far as the Square example is concerned) the "Visa 1111" or "Sign Here" portion. It would not be expected that a user would start and contain their signature on these two elements.
(n1) - Average 84% of total designated display area. Display area includes, in this case, the iPhone5's instructions and UI elements that give parameters to the signature area. As a visual example, on the above image, the signature does not extend above the line created under "$25.00" - and although it could've, the signature is "implied" to stay within certain boundaries. I've approximated this at an average of 84% the pixel space (93% of horizontal space, 75% of vertical space).
Edit: The arrow in the bottom-most picture points at whitespace and lists it as "75% of Vertical Space". Apply this to the "Identified" Container, not the whitespace.