Lots of good things mentioned so far.
I have built a tiny home were I plan to retire. I built it in my driveway and moved it to a few acres that I plan to retire on. I have had it in place for 3 years now. A month is my longest stay, most of my visits are 2 weeks at a time. So I cant give long term advise. My home is 144 sq ft with the ability to easily add on should I so decide.
Building anything is all about compromise. My reason for building tiny was based on cost. Not just the cost of building but heating, cooling and maintaining the building on a limited income as I age.
Nichol mentioned that a larger space can give the appearance of less mess. I would some what agree. A larger area can contain more clutter and still have usable space. A good example in my life is the dinning room and dining table in my 1900 sq ft home. I don't use my formal dinning room so that table becomes a catch all. Coats on the back of chairs, motorcycle helmets emptying pockets etc. etc. So my 12 x 14 foot dining room is a very large hallway with lots of surface space to fill without ever really being in the way. I still have to heat and cool all that area plus walk past it all to get any where in the house.
Joshua mentioned a well thought out space. I feel that's is the key to going smaller. Volume of space can easily compensate for poor design. A poorly designed kitchen with lots of counter space can still function but at the cost of more travel time between functions. And more space to heat cool and clean.
I have a background in woodworking and construction. I spent 7 years from planing stage to parking the house at its permanent home. I custom built pretty much everything. I knew this would be a major undertaking and I wanted to get it right. My first step was to really be honest with myself about how I use the space I live in. Where I spend the most time and what can I remove from that space without feeling like I am sacrificing a thing. I used a floor planing app I found on line and started measuring the space I use in my large home and arranged my needs in virtual space just see how much space I need. As it turns out that space can fit on a 8 x 18 trailer with a few bump outs. I am thrilled with the results, It feels like a warm hug every time I enter or just sit and enjoy it. It works well with 1 guest for short term stays. It would not be enough for a partner or one of my sons to be a permanent resident that's why I built with the plan for expansion. For my personal comfort level even if I had a partner I wanted to spend most of my time directly on top of I think I would want at least twice the space of my little home. More space if it were one of my sons.
All this just to say, If this is something you want to be permanent take a very close look at how both of you use your space. And build to that size. Rather than trying to squeeze your needs into what will fit on a trailer. If this is a short term solution, I would cut lots of corners to save for a more permanent solution. And make sure everyone involved knows its temporary and working towards a goal.
Just my opinion.