The cleric equivalent is pretty much Magic Weapon, which doesn't increase the weapon size. You could however cast it on a Greatsword, and end up with something functionally similar to a quarterstaff + shillalagh.
Ask your DM. Some will allow it, some will not.
As DM, would allow a cleric to learn Shillelagh as a 1-st level spell, and flame blade as a 3-rd level spell.
(general rule for me is +2 to spell level vs cross class spells, but have notice in pathfinder many class related spells are +1 )... Shillelagh is so low level, would keep it as 1st level, while i would up the Flame blade by a level...
Would require the Cleric to Independently Research both spells first. Spend 1 week per spell level in time, and 1,000 gp per spell level in cost.
Both spell are Unique to that character, and can not be tought to other cleric, Unless they also spend the time and money for "Independently researching the spell's".
PS = If a wizard, asked about these spell, would still have done shillelagh as a 1-st level spell, and flame blade as a 2-nd level spell, but still required the independently research.