I needing a new. battery just to run my Livescope and a Garmin fish finder on the dashboard what amp and group size do I need never used a lithium battery before

Look up the amp draw of whatever graph you’re using and the amps of the livescope. Multiply by how many hours you want to run it. Get a battery with at least 10-20% more amps than that

Does the group size matter or will a 50 or 100 amp matter as long as the formula you gave me works
Tks

Physical dimensions have nothing to do with the battery's capacity. It's the amp hour rating that matters.

Some of the mfg have two physical sizes for the same AH capacity.

My suggestion is 50ah. They are more affordable, smaller and lighter. Just make sure your charger can charge lithium batteries. You could go more ah if you need to have more capacity between charges.