A +1.5 spread is commonly seen in baseball betting, the standard runline for MLB. This spread means the underdog must win outright or lose by exactly one run to cover the spread. Alternatively, a -1.5 spread means that the favorite must win by at least two runs. Many baseball games are decided by fewer than two runs.
A good medium is 3% per play. For example, if you're starting with a bankroll of $100, you should risk $3 on every bet. If you're starting with $1000, you should risk $30 on every bet. Your unit size would be $3 or $30, respectively.