Darn! Think I typed a symbol that this forum read as a cutoff point...
Many females can breed at 40g without incident, but as this is not the maximum size for most females, it will put a stress on them...
Always remember that with young leopards (or any living thing), energy is first and foremost for growth (outgrow the size of as many predator species' mouths as possible). Later in life, energy is allotted to a novel event - reproduction. Energy used for growth is not available for reproduction, and vice versa.
So, if you have a small, younger (less than 10-12 mos.) female leopard, don't breed it until it starts ovulating that first year. Females often lay whether they've been fertilized or not; past the bit of stress the courting male puts on them, it seems a shame to waste those spent energy resources on dud eggs unless a female is so small (less than 40g) that the whole process would be a risk.
An important consideration is, have the husbandry practices allowed the female to reach prime size by the time she starts ovulating (it ain't all up to genetics) ?
Since the common practice is to breed for color traits over all others, some leopard strains have seen a reduction in size attained by the average animal. Some of these females never achieve a size that's safe for breeding. My normal practice is to wait till ~45g for most females. Waiting till 60g for females is overkill. With males, I wait till they're bigger, but that's not a health-related practice. Hope that helps.