Cytisus "Firefly", how to care for?

I was given as a present a pot of Cytisus "Firefly".

The instructions card says that after flowering it could be cut back to 20cm (8 inches), which is roughly how tall the stems are as it came (they are 9 to 10 inches).

The pot it came in is about 3 inches square and 4 inches deep. The compost wasn't completely dried out, but I gave it a good watering and put it outside.

What size pot do I need to re-pot it into?

What kind of compost does it like?

The person who gave it to me thought it looked dead, but I think it may have been cut back by the supplier before selling, and after flowering, so probably won't flower until next year?

I am considering getting a fairly big pot for it and planting flowering annuals in the same pot, which will be cut back at the end of the summer leaving the Cytisus to do its thing next April/May.

 

Hi looked up these 3sites they might help
Cytisus scoparius &Firefly& | Shrubs/RHS Gardening
https://www.rhs.org.uk/plants/31020/i-cytisus-scoparius-i-firefl…
Prefers well-drained, poor, neutral to slightly acid soils in full sun. Tolerant of lime but may become chlorotic on shallow chalk soils. Plant out when small, as brooms resent transplanting Propagation Propagate by semi-ripe cuttings in late summer or early autumn, or by hardwood cuttings

Cytisus scoparius 'Firefly', Broom 'Firefly' in …
https://www.gardentags.com/.../cytisus-scoparius-firefly/8007
Cytisus scoparius 'Firefly' type

Cytisus Scoparius 'Firefly' from Burncoose Nurseries
Cytisus Scoparius 'Firefly' from Burncoose Nurseries
Plant can withstand temperatures down to -15°C (5°F) Full sun Shelter from cold, drying winds

 

Thanks. I think I had read that bit about planting out while small, as brooms resent transplanting. Which is why I am considering getting a very big pot for it and in the first summer adding some annual flowers that get cut back at the end of the summer. Rather than transplanting it into only a slightly bigger pot on its own.