Choose how thick they are

Eyelash extensions come in different widths. There are two issues to consider. The first is: how thick and strong are my own natural eyelashes? You can only wear the width of extension that your natural eyelashes can support. The second issue is: what look am I going for? The thicker the extension lashes, the more pronounced will be the effect.

0.05mm and 0.07mm Extremely fine lashes, which are only practical for volume lash extensions.
0.10mm and 0.12mm Very fine lashes, which are suitable for thin or weak natural lashes.
0.15mm Suitable for fine to strong lashes.
0.18mm Good for strong lashes.
0.20mm Good for extra-strong natural lashes.
0.25mm Very noticeable lashes which will be too heavy for most natural lashes.

The crucial issue is that your natural lashes must be strong enough to support the lash width you choose. If the eyelash extensions are too wide (and therefore too heavy) for your lashes, your natural lashes may break off. It is a similar issue to the question of ‘How long?’ your extension lashes should be.

If you have fine-haired lashes, or thin or weak natural lashes, you need to keep to consider 0.10mm and 0.12mm as your most practical choice.

If your lashes are naturally thick and strong, then 0.18mm or even 0.20mm may be fine for you. You would have to have really think natural lashes to choose 0.25mm, and those lashes will look false.