I'm thinking maybe in this northern climate, Ichang papeda fruits do not have time to grow to full size. The region around Portland/Olympia has already entered into the colder rainy season.
Keep in mind the early Fall season here has cooler/colder average temperatures than New England (although the temperatures show more stability with fewer very cold days).
If the fruits have only reached this size by this time in the year, then it is difficult to imagine them being able to grow any bigger.
This is from a big tree that should be fully mature enough.

I do not think it is the northern climate. My climate is cooler than Portland and has significantly fewer average hours of sun per day, yet the fruits grow to normal size and have no problem reaching maturity.

Would like to ask regarding the precocious, how long will take to fruit from seed?
Also would be interested what is minimum temp. it can withstand and what is best temp. to overwinter (could be overwinter in room temp. around 20C)?

I would also like to know what minimum temperatures they can take for a longer period of time. So far, I have been reluctant to buy a finger lime because I fear I will kill it in its first winter anyway. I overwinter my potted Citrus in a cold but just frost-free greenhouse.

 19F/-7C seems weak. Any true Citrus ichangensis should be able to take more. I think Ilya said that Citrus ichangensis breaks dormancy quickly and is subsequently more vulnerable to (late) frosts and can be easily damaged at much higher temps than normal. I have never had this with my Citrus ichangensis but it is grafted on Poncirus. There is a fella near Cologne (GER) who has a seed-grown, fruiting tree which he claims is very coldhardy (also, the fruit are rather large and nearly seedless).

Last week, I planted this Citrumelo Nr. 82, that I grafted myself, in my grandma's garden. My own garden is very small and she let me use this corner . Citrumelo 82 is a selection made by Eisenhut nursery here in Switzerland for those who don't know this cultivar. Pics and description can be seen here: https://www.forum-agrumes.com/viewtopic.php?f=13&t=948

I just wanted to show you this photo. I always enjoy it very much when my ichangensis produce new growth.

Yuzu x keraji certainly does sound promising.

Thanks for the link. I don't like the many names either but at least it is becoming available in Europe.

This could be an interesting cross and probably pretty coldhardy but as far as fruit quality goes just get the right Yuzu, which is excellent.

Seems to be only monofoliate now, right?

Quick update. The fruit is actually good to eat when the rind is still greenish so I picked two today.
 

For comparison a large(ish) navel orange. Are these spots frost damage?

I still have some fruit on my ichangensis x sinensis. We are expected to see -4/-5C the coming nights.
They have already seen a few -2/-3C and still look pristine.
Will they wrinkle or fall off or something like that when damaged by frost?

Thank you Socal, for posting that image of my tree. 
I harvested the fruit on that in mid-November.  About 6 lbs.  They range in size from sub-ping-pong ball to medium size Satsuma.  The flavor is good, but rather sour.  Ripeness varies considerably.  The ripest ones have 'squishy' peels and peel rather easily, though not so easily as Satsumas.  Since snow flurries are occurring, I think I'll stay indoors and use the time to do a Brix and Acid level reading this afternoon, using the smaller ones.



I apologize for the blurriness of the image, but I ain't no photographer.  The fruit above the ruler are some of the Citsumas; I've eaten the largest few, and juiced a few others for a Chinese dish.  The three fruit on the bottom of the image are typical LA Early Satsumas, to provide scale, along with the blurry ruler.

Thanks for your comparison, Jim! May I ask if your Prague is a regular bearer? Has it ever had trouble fruiting?