Size variables/parameters could be updated to use the `size_t` type

[thnkslprpt]

Checklist

• I reviewed the Contributing Guide.
• I performed a cursory search to see if the bug report is relevant, not redundant, nor in conflict with other tickets.

Describe the bug
There are several variables/parameters in the MD app representing size that could be updated to use the size_t type.

Reporter Info
Avi Weiss   @thnkslprpt