If you ground one lead, and short the other lead to that ground (Or
connect both leads to ground).. would this trip the breaker or since
it is an isolation transformer and the current is limited by the core
size.. then it can't trip the breaker?
The current is not limited by the core size, but rather by the resistance of the wires (at a given voltage level). If you short the secondary, the current will increase in both secondary and primary, which will cause the transformer to heat up and, likely, to burn up. If it is a high power transformer, the current in the primary could become high enough to trip the circuit breaker.
If you touch any lead of the isolation (step down) transformer, you
are not supposed to get electrocuted.. but if you touch both leads..
would you be electrocuted or still not?
If the secondary voltage is high, say, more than $50$VAC, it is dangerous to touch even one lead, because the second lead could be grounded, in which case the first lead would be at 50VAC relative to ground. If you touch that lead with one hand and touch a grounded object with the other hand, a dangerous current could flow through your body.
If the secondary voltage is low, say, $12$V, you can touch one lead or both leads with no consequences, since the worst case current through your body will be very low. Still, I would never deliberately do it, because a mistake or a broken transformer could have lethal consequences.