Nelson Bench 48" or 60"  

48" vs. 60"
I've been building a slat bench and opted for 48". The length seemed about right for use as a one-person bench, and in my small apartment. I can't see more than one person fitting on it, however. As a coffee table, it's just about the right size for me, but it's too small to support a TV, etc.
Are you planning on using it for either a bench, a table, or both?

multi-use
That's the problem with multi-use furniture. When I first get it, it will be used as a coffee table, but eventually to be later used as a bench against a wall or at the end of a bed.
It will definitely not be used to hold a TV or any of the sort.
However, something else I am thinking about is the future with it. Right now I live in a house with relatively small spaces. Based on that understanding alone 48" might be ideal. However, I will not be living in the house forever and eventually move into a larger house.
I guess the real problem might be, should the size (if this makes any sense at all)of this multi-use bench reflect proportionally in acknowledgement to its surroundings? Meaning, how should it proportionally size up to the end of a bed, sofa, or against the wall.
In detail:
Should the bench be just as long as its associated sofa or shall it be a little smaller.
Should the bench be just as long as the width of its associated bed or it that proportionally incorrect.
Should the bench size be determined by the size of its surrounding space, small space calls for small bench?
For me, this bench will be intended to be used for both functions, proportion is more at question than anything else.
Not the bench itself, but how it measures up to its surroundings I have yet to make a decision on.

either one
I live in a very small space but my Nelson is 60". It doesn't bother me and it provides table for anyone on the sofa. There are no sde tables so this works well. However, visually the shorter would look good too. I opted to have the bench be closer in size to the sofa. I have a full size bed and the 60" is too wide for the foot. Depends on what size bed you have. Honestly I think I would have been happy either way. For coffee table use it is still my gut feeling to get something closer to your sofa dimensions.
There are some things I regret about this bench being my coffee table. One, is that I feel like a prude having to remind people to use coasters because it is wood. The other is that the slats force you to do a bit of a balancing act with the coasters and drinks. Although I can't think of a table I would rather have aesthetically for my space, sometimes I wish I could find something I like as well that emplyes the use of glass or laminate.
good luck!