In Unity particle effects, if you do not want to set loops and do not change the starting life, you can make particles persist in one of two ways:
1. Use the Duration property of the Particle System. This property specifies the duration of the particle system. Once the duration of the particle system is over, the particles will stop firing. Setting the Duration property to a high value allows the particles to persist for a long time.
2. By controlling the emitter of the particle system in the script, it is possible to make the particle persist without changing the initial lifetime of the particle system. For example, you can use the emit parameter to emit a single particle and then emit the next one after the particle is gone to maintain the continuity of particle existence.
If you want to change the initial lifetime without changing the particle size, you can do this by adjusting the scale of the particle. In the Renderer property of the Particle System component, you can find the Material property, in the Material property you can find the Texture Sheet Animation module, in this module you can find the Tiles property, Set it to 1x1(using only one texture) and then set the Frames Per Second property to 0 so that the texture of each particle can be fixed and the Size and Lifetime of the particle can be controlled by adjusting the Start Size and Start Lifetime properties. By adjusting the Start Lifetime attribute, the particle lifetime can be controlled, and by adjusting the Start Size attribute, the particle size can be controlled. The scaling will not affect the size of the particles because they are already fixed.