Hi Cinderella,
One of the links in the “read first post” is mattress firmness/comfort levels in post #2 here which has links to some very general guidelines that can give you some insights into different mattress designs and how they apply to different body types and sleeping styles but your own personal testing is the only way to “translate” very generic information into specifics that apply to you. What you are looking for is PPP (Posture and alignment, Pressure relief, and Personal preferences) that is specific to you based on your own personal testing and experience and the theory of how to get there is secondary to the results of your own testing. Your body is a much more reliable indicator of the suitability of a mattress than any “theory at a distance” and I would only use generic guidelines as a reference point or as a place to start … not as a “target”. I would also never use reviews or recommendations based on other people’s experiences on a mattress to decide on which is the best mattress for you (see post #13 here about mattress reviews).
I personally wouldn’t assume anything and your weight and height aren’t nearly as important as how a mattress feels and performs for you. Once you’ve read the basic information then you will be more familiar with the idea that there are different types of softness/firmness that apply to different parts and layers of the mattress (see post # here) so in combination with your personal testing you will be better able to assess whether a mattress is too soft or firm in the support layers (primary support and alignment that “stops” your pelvis from sinking down too far) or the top comfort layers (primarily pressure relief and secondary support that fills in the gaps in your sleeping profile) rather than only going by the more subjective “overall feel” of the mattress. In other words you will be aware that changing the softness and thickness of the comfort layers has a different effect from changing the softness / firmness of the support layers and will have a better sense of what to look for if a mattress you test isn’t quite right and you need something different. Once you have “enough” information that you can use in practice … then trying to pre-design your mattress based on specs will usually take you over the line into “paralysis by analysis” and can lead to unsuitable choices just as easily as not having enough information in the first place. If you trust your body (not the specs), test mattresses carefully using the more objective testing guidelines suggested, connect with knowledgeable and experienced manufacturers or retailers, and always remember that PPP and the quality/durability of the materials (especially in the upper layers) are all that is really important, then your odds of success are very high.
Post #6 here (centered on Lehigh Valley and which also links to other nearby forum lists) would include the better options and possibilities that are closest to you that I’m aware of. I would also keep in mind that whether or not a retailer or manufacturer is on the list that your ability to find out the specific details of what is in any specific mattress you are considering is the only way to know the quality/value of a mattress and make meaningful comparisons with other mattresses.
If you do decide to go in the direction of exchanging your mattress at Sleepy’s then post #2 here includes includes links to other forum threads with suggestions and ideas to forum members that were in a similar difficult situation where they needed to exchange a mattress at a store where there was little to nothing available that had good quality or value.
Phoenix