I added adj-no to sense 1 in 2016 but there's little evidence for it in the n-grams (looking at the top 〜の terms).
I don't think adj-f is needed. Those examples are really just compound nouns.

@@ -18 +17,0 @@
-<pos>&adj-no;</pos>
@@ -24,2 +22,0 @@
-<pos>&adj-f;</pos>
-<xref type="see" seq="1605510">痩せる・1</xref>

GG5, 中辞典, etc.
痩身	476624	99.5%
そう身	2263	0.5%

I don't mind having a couple of synonyms, especially when they are in the JEs.

@@ -9,0 +10 @@
+<ke_inf>&sK;</ke_inf>
@@ -18,0 +20 @@
+<gloss>slender body</gloss>

痩身	476624
痩躯	5057	
長身痩躯	2833


Reverso, endless examples, almost always glossed as "slim" or "slimming"
https://context.reverso.net/translation/japanese-english/痩身

痩身や、肥満を軽減するため  to lose weight and reduce fat,
貴重な痩身援助です。 ... aids slimming.
痩身剤 slimming agent
痩身エステ slimming esthetics!
健康的な痩身 "healthy slimming"
RFレーザー技術で痩身サービ SLIMMING WITH RF LASER TECHNOLOGY
あなたのウエストラインを痩身. slimming your waistline.

etc.

Other adj-f justifications:
痩身エステ	28147	  
痩身法	26032	  
痩身効果	16681	  
痩身術	14748	  
痩身用	13361	  
痩身方法	11040	  
痩身メニュー	9748	  
痩身用食品	8836	  
痩身カプセル	6841	  
痩身コース	6784	  
痩身ダイエット	6258	  
痩身カプセル試し	4879	  
痩身機	4705	  
痩身医学	4437	  
... etc....

The xref is quite uncommon, and probably unhelpful to point at without significant nuance explanation. (comments at the xref, also edited)

I feel like pushback will come on removing "lean figure".  My argument is that "slim" is almost always the right J-E gloss. (or "weight loss" if the text is more elaborately massaged).

Sure, you won't get this entry if you search "lean", but you won't get it if you search "thin" either.  

The sankoku definition are:
1.  やせたからだ
2. 「ふとったからだを」やせたからだにすること

Either way, you have to have weight and then lose it.  For "lean" a person can always be lean.  For "slim" I grant that it's ambiguous, (I think someone can always be slim if they've been healthy all their lives), but there is a strong exercise/weight loss/health connotation to "slim" which seems to match this term. Having "lean" on a gloss here, to me, makes it appear less-specific than it actually is.

Google image search for "痩身" kind of says everything, I think.

@@ -18 +17,0 @@
-<xref type="see" seq="1835410">痩躯・そうく</xref>
@@ -20 +18,0 @@
-<gloss>lean figure</gloss>
@@ -23,0 +22 @@
+<pos>&adj-f;</pos>
@@ -24,0 +24 @@
+<gloss>slimming</gloss>

G n-grams 476624

@@ -5,0 +6 @@
+<ke_pri>spec1</ke_pri>
@@ -11,0 +13 @@
+<re_pri>spec1</re_pri>