The diameters of electrical wires are designated by their “AWG” number. Smaller numbers are thicker wires.

AWG	Diameter (inch)	Diameter (mm)	Resistivity (Ω/km)	Max. current (A)
6	0.162	4.1148	1.295928	37
7	0.1443	3.66522	1.634096	30
8	0.1285	3.2639	2.060496	24
9	0.1144	2.90576	2.598088	19
10	0.1019	2.58826	3.276392	15
11	0.0907	2.30378	4.1328	12
12	0.0808	2.05232	5.20864	9.3
13	0.072	1.8288	6.56984	7.4
14	0.0641	1.62814	8.282	5.9
15	0.0571	1.45034	10.44352	4.7
16	0.0508	1.29032	13.17248	3.7
17	0.0453	1.15062	16.60992	2.9
18	0.0403	1.02362	20.9428	2.3
19	0.0359	0.91186	26.40728	1.8
20	0.032	0.8128	33.292	1.5
21	0.0285	0.7239	41.984	1.2
22	0.0253	0.64516	52.9392	0.92
23	0.0226	0.57404	66.7808	0.729
24	0.0201	0.51054	84.1976	0.577
25	0.0179	0.45466	106.1736	0.457
26	0.0159	0.40386	133.8568	0.361
27	0.0142	0.36068	168.8216	0.288
28	0.0126	0.32004	212.872	0.226
29	0.0113	0.28702	268.4024	0.182
30	0.01	0.254	338.496	0.142
31	0.0089	0.22606	426.728	0.113
32	0.008	0.2032	538.248	0.091
33	0.0071	0.18034	678.632	0.072
34	0.0063	0.16002	855.752	0.056
35	0.0056	0.14224	1079.12	0.044
36	0.005	0.127	1360	0.035
37	0.0045	0.1143	1715	0.0289
38	0.004	0.1016	2163	0.0228
39	0.0035	0.0889	2728	0.0175
40	0.0031	0.07874	3440	0.0137

Source: https://www.powerstream.com/Wire_Size.htm.

Resistance is for copper wire. Most other metals are worse conductors.

Maximum currents are guidance only. Different sources list different numbers. For instance: https://www.engineeringtoolbox.com/wire-gauges-d_419.html. The thing that actually matters is how much voltage drop (and heat generation) you can tolerate.

There are many voltage drop calculators on the web. For instance: https://www.rapidtables.com/calc/wire/voltage-drop-calculator.html.