If you still see big flames after installing the pack, the cause is usually pack order, a missing pack, or another source of textures overriding it.
Check the Resource Packs menu
1. Open Options > Resource Packs.
2. Make sure NO FIRE 2.0 is in the Selected list on the right. Its custom pixel-art icon makes it easy to spot.
3. Look for other packs above it that change fire, such as full texture packs or other PvP packs. Drag NO FIRE 2.0 higher than them.
4. Click Done and let the textures reload.
If the pack is not listed at all, open the resourcepacks folder with the Open Pack Folder button. Confirm the pack file is directly inside that folder and not buried in an extra subfolder. Then reopen the menu.
Other things that can override it
- Server packs: Some servers send their own resource pack, and it loads on top of yours. If fire only looks normal on one server, that server's pack is probably replacing the fire textures.
- Shaders: If you run shaders through a loader such as Iris or OptiFine, test with shaders off. If the fire looks correct then, the shader setup is changing how fire is drawn.
As a final test, select NO FIRE 2.0 as your only pack and light a fire with flint and steel. If it works alone, add your other packs back one at a time to find the one causing the conflict.