The phrase "are often so small" is correct and usable in written English.
It can be used to describe something that frequently has a small size or dimension.
Example: "The details in the painting are often so small that they can only be appreciated up close."
Alternatives: "are frequently quite tiny" or "tend to be very small".
They are often so small that my phone has trouble focussing given the close range of my subject".
Rural schools are often so small that one instructor must be responsible for multiple subjects, and sometimes multiple grades.
The cracks discovered by Amtrak inspectors are often so small that they are invisible to the naked eye.
At startups in particular, teams are often so small that you'll find yourself working directly with the founders of the business.
But the boats are often so small that the carabinieri can't spot them even on their radar screens, and a horrific number of people drown or die of thirst en route.
Our abodes are often so small, and our time outside of them so large, that much of what would be alarming on the streets of Peoria, Illinois is unremarkable here.
While it did induce the ice crystals, the drops of water that fell from seeded clouds were often so small that they evaporated before they reached earth.
"However, the brain damage is often so small and subtle that most people don't notice it". Vascular dementia gets confused with normal aging since symptoms can mirror "senior moments," like forgetting a name or just-learned information.
Since the type used in ads, magazines and books is often so small that even those with perfect vision can't read it, I think that the problem is less the age of the designers than their attitudes.
In the TEM, for example, the field of view is often so small that no aperture with perfectly abrupt edges exists, in particular not at atomic resolution.
Many-valued logicians note that the error they are imputing to classical logic is often so small that classical logic can still be fruitfully applied.
