Help convert this C++ to Delphi Measuring size of Memory allocated to a Type

This is the C++ code
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ImageBuff = (uint8*)malloc(ATGetImageBufferSize());
memset (&sEnrollResults, 0, sizeof(tsAT_ENROLL_RESULTS));
sEnrollResults.uiStructureSize = sizeof( tsAT_ENROLL_RESULTS);

Delphi Code
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Reallocmem(pImageBuff,ATGetImageBufferSize);
//e.g ATGetImageBufferSize returns 18090 so pImageBuff is sized to 18090.
If I send sizeof(pImageBuff) I get 4. I want to measure the size of the
allocated memory so that I can get 18090. Please assist am stuck!