Use size_t for 'size' variables

[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
Most variables representing sizes in FM use size_t.
There are a few that do not follow this custom and could be transitioned over.

Code snips

FM/fsw/src/fm_child.c

Line 1396 in 2be3511

int32 WriteLength = sizeof(FM_DirListEntry_t);

FM/fsw/src/fm_cmd_utils.c

Line 161 in 2be3511

uint32 FM_GetFilenameState(const char *Filename, uint32 BufferSize, bool FileInfoCmd)

Expected behavior
All size variables that can use size_t should do so - size_t is more expressive and more consistent with the relevant coding guidelines.

Reporter Info
Avi Weiss @thnkslprpt