In a .bat file, how can I test available disk space?

I suggest using a PowerShell script for that.

wmic returns the size in bytes, batch is limited to number comparisons of signed 32-bit integers, far below 100GB. You would have to truncate the number first.

PowerShell

if ((Get-WMIObject Win32_Logicaldisk -filter "deviceid='C:'").FreeSpace -gt 100GB){
  "yes enough free space"
} else {
  "not enough free space"
}

I have to extend the above information about number comparison in batch.
If numbers are contained in equal length strings they can be validly compared as strings.
A string comparison takes place character by character, a leading space/zero is less than 1.

At the moment the largest hard drives size is 10TB = 10995116277760 bytes taking 14 decimal places - the following batch uses 15 places for all numbers.

@Echo off
Rem        543210987654321
Set "Blank=               "
Set "GB100=   107374182400"
Set "TB_10= 10995116277760"

for /f "tokens=2" %%A in (
  'wmic LogicalDisk Get DeviceID^,FreeSpace ^| find /i "C:"'
) Do Set "FreeSpace=%Blank%%%A"
Set "FreeSpace=%FreeSpace:~-15%"

Echo FreeSpace="%FreeSpace%"
Echo    100 GB="%GB100%"

If "%FreeSpace%" gtr "%GB100%" (
  Echo yes enough free space
) else (
  Echo not enough free space
)

Sample output:

> Check-FreeSpace.cmd
FreeSpace="   101606346752"
   100 GB="   107374182400"
not enough free space

> Check-FreeSpace.cmd
FreeSpace="  1181504520192"
   100 GB="   107374182400"
yes enough free space

The simplest way is to look at the last line in dir output

dir /-c /w C: | find "bytes free"

You can also get the size value directly by getting the 3^rd token in the output

FOR /F "tokens=3 USEBACKQ" %%F IN (`dir /-c /w C:`) DO set "size=%%F"
echo %size%

However it's not reliable as the string may be different in another language. And also note that sometimes you have a lot of freespace but can't write any more files because you've reached some limits like maximum file size or maximum number of files in a volume/directory...

The most correct way is

wmic LogicalDisk where DeviceID='C:' Get FreeSpace /value