Hi CatCuddler,
It really is a personal preference, and over the years I’ve been doing this I’ve had petite individuals choose anything from very soft to very firm. And while people with a lower BMI “tend” to favour items in a more “plush” range, I would also keep in mind that there are also no “standardized” definitions or consensus of opinions for overall firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer.
Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. This is all relative and very subjective and is as much an art as a science. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here and what it links to for more information on mattress “softness” ).
If you can’t test a mattress in person, then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (fast or slow response, resilience, firmness etc.) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. They will know more about “matching” their specific mattress designs, options, and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
Ultimately, the only way to know for certain whether a mattress feels soft or firm to you or whether it fits your criteria will be based on your own personal testing or experience.
Phoenix