You really don't want to be cutting these plants back while they are getting established. At most I'd trim the flowers off if they bother you.

Otherewise once you get them established they respond nicely to a cutting back of a 1/3 every other year. As they get older a more severe pruning may be required to maintain density.

In general spirea are very low maintenance shrubs. Just let em be.

ignore them for 3 years...

and they will get as big as you want ...

its counter intuitive to go lopping off 1/2 the plant.. so it grows bigger ...

its a matter.. of growing the root mass... sufficient to support what you dream above ... the downside being.. you cant see it doing any of the work ...

i am a firm believer.. in the notion.. that fert will not make it happen any faster ...

my guess... would be.. its going to throw some two to 3 foot branches.. by fall ...

i am not sure you have the best sun.. for these full sun plants ... so they might not be as vigorous as their potential .... but they will not suffer... it will just take a year or two longer ...

ken