Hi my name is ***** ***** I can assist you with your question.
To be clear, is the slat base the large, horizontal area the mattress rests on?
If so, I believe 1 to 1.5 inches larger than the mattress size in both width and length
would work perfectly. I'm here in the US and unfamiliar with metric, but I think you would
want your slat base to be around 162.45 x 202.45, that's adding an additional 1 inch both in width and length.
Or, for a less snug fit, add 1 1/2" (3.81 cm) to both length and width. I don't think you would want it much larger than this except for fashion purposes.
If the slat base is solid wood, your measurements could be anything, 1 to 5+ inches. However, I wouldn't make it so large that it's hard to get out of bed in the morning.
Remember I cannot "see" the bed you are designing so the vision of the finished bed in my head may be different than yours. So to be safe you would want to send me a photo of what you are trying to replicate and I can be more definite. The storage bed sounds like a great idea!!
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