The diameter of syringe needles is designated numerically with a standardized gauge. Small numbers correspond to large diameters:

Needle gauge	Outer diam. (mm)	Inner diam. (mm)
7	4.572	3.810
8	4.191	3.429
9	3.759	2.997
10	3.404	2.692
11	3.048	2.388
12	2.769	2.159
13	2.413	1.803
14	2.108	1.600
15	1.829	1.372
16	1.651	1.194
17	1.473	1.067
18	1.270	0.838
19	1.067	0.686
20	0.9081	0.603
21	0.8192	0.514
22	0.7176	0.413
22s	0.7176	0.152
23	0.6414	0.337
24	0.5652	0.311
25	0.5144	0.260
26	0.4636	0.260
26s	0.4737	0.127
27	0.4128	0.210
28	0.3620	0.184
29	0.3366	0.184
30	0.3112	0.159
31	0.2604	0.133
32	0.2350	0.108
33	0.2096	0.108
34	0.1842	0.0826

Source: https://blog.darwin-microfluidics.com/syringe-needle-gauge-table/.