daijr: 栗の実ぐらいの小石。

I think this is sufficient for sense 2.

@@ -16 +16 @@
-<gloss>construction stone (usu. 10-15cm diameter)</gloss>
+<gloss>construction stone (approx. 10-15 cm in diameter)</gloss>
@@ -21 +21 @@
-<gloss>rounded river stone about the size of a chestnut</gloss>
+<gloss>chestnut-sized pebble</gloss>

Koj, Daijr/s, WWW images
栗石	4931	97.5%
くり石	125	2.5%

GG5 has "cobblestone" but I think it's wrong. Trimming and putting the main modern use first.

@@ -8,0 +9 @@
+<ke_inf>&sK;</ke_inf>
@@ -15,2 +16 @@
-<misc>&rare;</misc>
-<gloss>rounded river stone about the size of a chestnut</gloss>
+<gloss>construction stone (usu. 10-15cm diameter)</gloss>
@@ -20,3 +20,2 @@
-<xref type="see" seq="1207770">割栗石・わりぐりいし</xref>
-<s_inf>traditionally river stones, now more commonly crushed rock</s_inf>
-<gloss>construction stone (usu. about 10cm ~ 15cm in diameter)</gloss>
+<misc>&rare;</misc>
+<gloss>rounded river stone about the size of a chestnut</gloss>

sankoku says "rounded". Clearly not, mostly because 栗石 can apply to 割栗石 of the same size, which is jagged.
1.
栗の実ぐらいの小さな丸い石。
2.
河原にある、径一〇〜一五センチぐらいの丸い石。基礎工事用。

daijs. 土木建築用の直径10～15センチぐらいの石。多く基礎材として使う。

https://forum.wordreference.com/threads/裏栗石.2427088/
I'm translating a document about the architecture of Himeji Castle and the term 裏栗石 came up
...
I guess the English term is stone fill. But you need additional explanation to explain the Japanese dry stone technique. 裏栗石 is a river stone around 10 to 30 cm in diametre (or larger ones broken to the size; that's the definition of 栗石) as used to fill the space between the rocks on the surface and the inner earth mound.

https://www.token.co.jp/estate/useful/archipedia/word.php?jid=00016&wid=29121&wdid=01
「栗石」とは、丸みを持った径が15センチ以下の石のこと。

sankoku:
1　栗の実ぐらいの石
２　石垣や地盤固めに用いる、直径十数センチの石。割りギリ石。


Google, what size is cobblestone? 75-250 mm

https://southbaymaterials.com/products/cobbles#:~:text=Cobble landscaping rocks are naturally,ponds, and as natural accents.


https://www.aisf.or.jp/~jaanus/deta/k/kuriishi.htm
Also pronounced guri-ishi. Chestnut stones. Small stones (10-30cm in diameter) that were used as packing or backfill. Larger kuri-ishi were used to fill the cracks and spaces among ordinary stones *hira-ishi 平石, or the face of a castle's stone wall *ishigaki 石垣. Smaller kuri-ishi were used to back-fill the main stones, of a castle wall. Layers of chestnut stones might be one meter thick. Chestnut stones used as backfill were also called uragome-ishi 裏込石 (see *uragome 裏込), literally meaning stones to pack behind the main stones, or tsume-ishi 詰石 (packing stones).

I can't figure out what this *is*, but I am pretty confident that it is not cobblestone.

It's not this:
https://sslandscapesupply.com/product-tag/cobblestone/

There are more flexible definitions of "cobblestone" that might overlap, but they aren't what one would usually imagine.

栗石 more often than not these days seems to refer to 割栗石 wish covers a wide range of crushed rock/stone.  The main characteristic of 栗石 is that it always appears in the context of construction (particularly historic excavations, walls, foundations, etc.).  Bigger than gravel, *smaller* than cobblestone is one definition I found.

But the size isn't even fixed.  Here someone just says "50cm幅の栗石が発見され", in an old wall. Here it may be referring to its quality as a large, naturally rounded river stone.
https://twitter.com/jcvd2000/status/1276290833302487040/photo/1

(The twitter poster may simply be wrong).  

What really makes it a 栗石 seems to me to be that someone built something out of it (or that someone is imagining its use as a building material).

Here you can buy  small, medium ,and large 栗石, the largest advertised as 最長対角線が1000mmを超える庵治石。 (庵治石 特大栗石)
https://ajirock.jp/ajiishi/ex_large_guristone

It's as if the word has lost all nuance. The best gloss is usually just "stone(s)". 

I really wanted to do away with "cobblestones"- especially since it's not at all clear how "crushed cobblestones" = "crushed rock" (I don't think 割栗石　is the result of crushing 栗石, but rather the creation of 栗石 by crushing larger rocks), but I haven't found an adequate replacement.

@@ -15 +15,9 @@
-<gloss>cobblestone</gloss>
+<misc>&rare;</misc>
+<gloss>rounded river stone about the size of a chestnut</gloss>
+</sense>
+<sense>
+<pos>&n;</pos>
+<xref type="see" seq="1207770">割栗石・わりぐりいし</xref>
+<xref type="see" seq="1207770">割栗石・わりぐりいし</xref>
+<s_inf>traditionally river stones, now more commonly crushed rock</s_inf>
+<gloss>construction stone (usu. about 10cm ~ 15cm in diameter)</gloss>