What size adult tricycle? I am 5 foot tall (152 cm) and have a 25 inch (63.5 cm) inseam.

I am unsure as to which size of wheel/frame to buy.  Also, recommendations for adult trikes, no more than $400 would be appreciated.
What size adult tricycle?


Sizing a bike is about getting a bike that fits your height and body proportions.

For an adult tricycle frame and wheel sizes are not a factor in fitting the vehicle to the person like they are with bicycles. Small wheel sizes on a trike help the rider get on or off the bike. Larger wheel sizes generally aid in riding longer distances and/or heavier loads.

All of the rider size adjustment for a trike will come in how high or low the seat will go and how the handlebars are adjusted.

Height recommendations are hard to find on trike specifications. I found one for the Schwinn 26" wheel trike. 


  suggested rider height is 5'4" to 6'2" and the recommended weight limit is 300 pounds.
  From Amazon product description  


Note, it says "suggested". I'm never confident that much effort - actual science - is put into "suggestions". According to the information provided this trike would not work for you. As I said - I had a hard time finding any height recommendations for the other trikes.

Since you are 5" tall riding before you buy is critical. 

Since an adult tricycle are supposed to be "one size fits all" how do you choose one?

First
One size does not "fit all". At best one size will fit a range of average sizes.  

Since you are not averaged size you will need to be careful to find something that works for you.  

The key is to only buy something you have test ridden.
A few things to look for on the test ride:
- Will it be easy to get on?
- Will the seat go low enough to be comfortable?
- Will the handlebars adjust to be comfortable?
- How easy is it to pedal?

If you are working with a bike shop and you have a trike that is close to fitting sometimes mechanics can swap parts or configure the trike to fit.

Second
My quick survey of the adult tricycle market indicates that you can get one, three, six, or seven speed trikes.
If you will be riding on very flat terrain one speed might be enough. If you will be riding very short distances one speed might be enough. Gears make it easier to get up hills, even small ones. Once again, the test ride is critical.

Third
There are other features to consider:
In no particular order - not a complete list
 - Folding? Do you need to be able to load it into a large vehicle?
 - Fenders? Will you be riding in bad weather?
 - Cargo capacity / basket? There are industrial trikes built to handle larger loads.
 - Seat comfort - try before you buy.

Fourth
Assembly - most of these come with "some assembly required".
Factor your mechanical skills or the availability of help into your buying decision.

How well a trike, or bike, is assembled has a direct relationship on how well a bike rides, how long it lasts and how safe it is to ride.

One more thing.
Evaluate if this purchase is worth the cost.  


List the benefits of having a trike 
Weigh the benefits against the cost.  


Calculating the cost  


How many times each month will you ride?  
Divide the cost of the trike (ignore assembly cost and maintenance for now) by how many times you plan to ride (be optimistic, it's a rough calculation) 


For example:
If you spend $400 and you ride one per month you are spending $33 per ride to have a bike for the first year.
If you ride once each week that cost drops to $7.60 per ride for the first year.

In either case the benefits of having the trike might make it worth the cost but it's something to take into account before buying.