https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1050857565
kanjigen:
［拳大］
ケンダイ
握りこぶしぐらいの大きさであること。

i'm tempted to write this こぶしだい off as A+B but it's in my IME and けんだい is worth recording

@@ -3,0 +4,3 @@
+<k_ele>
+<keb>こぶし大</keb>
+</k_ele>
@@ -11,0 +15 @@
+<re_restr>拳大</re_restr>

saw it used by a native in a sentence like this: 拳大の大きさ. The reading is bound to be controversial: 学研漢和大字典 and 漢字源 say it's kendai, but the internet (search for "拳大" "こぶしだい"), as 
well as my own near-native hunch wants to read it kobushidai. There's an entry for 拳大の石 (fist-sized stone) in WI2 (without the reading) but I don't think that's a special enough construct to 
warrant its own entry.