Electricity bolts are very fast, so in the first frame it should be at its maximum size and then it can dissipate slowly.
I usually make electricity starting with sharp angles and then it gets softer while it dissipates. Even though in the real world electricity almost never looks this sharp I found out that it looks better when animating if it does.
When I make electric stuff I usually add smoke too. If you want, I have a smoke tutorial here: https://www.patreon.com/posts/smoke-animation-6851636