Calculating subpanel breaker size
Feeder sizing isn't so simple
The minimum rules for service and feeder capacity planning are found in NEC Article 220; while they don't account completely for your usecase, we can use a modified version of those procedures here.  We start with the 550sf you gave, and multiply it by 3 to get the lighting and general receptacle load as-if this was part of its parent dwelling unit, giving us 1650VA (volt-amps, similar to watts).  We then multiply this by the .35 demand factor for dwelling unit general loads over 3000VA before adding 3000VA to account for the heavy tools circuit and 7500W/VA for the heater as per NEC 220.51.  Note that while the demand factor and the heater are both computed as per NEC Article 220 rules, the 3000VA for the tools circuit is an allowance (similar to a pair of Small Appliance Branch Circuits) for the heavy tools circuit due to the unavailability of details about the actual loads found there.
This gives us 11078VA, to which we add the dust collector load.  Since we are dealing with a UL listed appliance here, we start with the 20A nameplate amp rating instead of using the tables to look it up, although it doesn't matter in this case anyway since Table 430.248 gives us 20A for the full load current of a 2HP, 115VAC motor.  From there, we multiply by 1.25, per NEC 430.24 and 220.50, to get us 25A, which when multiplied by 115VAC gives us 2875VA, for a total load of 13953VA.  We then divide this by 240V, giving us a 58A minimum for our feeder ampacity.
While it's possible to wire this using 6AWG THHNs in a conduit or a 6-6-6-6 copper SER cable, that gives us 65A, leaving us little margin for an outbuilding. In NM, we'd have to use 4/3, giving us a 70A feeder instead, but neither NM nor copper SER are cost-effective ways to wire the circuit, at $3.5 to &gt;$4/ft vs the $2.5/ft of materials that using 6AWG THHNs with a 10AWG ground in 3/4&quot; ENT achieves.  Nonetheless, we can do better than that by going to aluminum.  A 1-1-1-3 aluminum SER cable goes for a mere $1.67/ft or so, and gives us 100A to the shop panel; we can upgrade this to a 1/0-1/0-1/0-2 SER cable to give us a 120A feeder (using a 125A breaker) for just under $2/ft.  If that's still too much cost, we can go with a 4-4-4-6 aluminum SER cable for just under $1/ft, but that puts us back at that 65A figure (with a 70A feeder breaker) from the 6AWG THHNs in conduit or 6-6-6-6 copper SER cable mentioned above.
GO BIG OR GO HOME
Now that we've decided on the size of our feeder, we need to figure out what size subpanel to put in, and this is where folks tend to get penny-wise and pound-foolish.  You see, ripping out a panel to replace it with a larger one is a big pain in the rear end, and far costlier than what you'd save by using a smaller panel.
Since this is in the same structure, we don't need to worry about a main cutoff (main breaker), nor a second set of ground rods, but we do need a set of separate grounding bars fitted to the panel to land equipment grounding conductors on.  Many main lug panels come with these factory fitted, but if yours doesn't, you'll need to buy the correct bars and add them yourself.  In either case, you'll want a large panel: a 24-space or 30-space, 125A panel is not at all out of place here, and if you feel like splurging, you can upgrade to a 40-space or 42-space, 200A panel for not that much more.  Just make sure that your panel's bonding screw or strap gets pulled out when you install it!
Sidebar: that dust collector may need a bigger breaker
There's one other issue with your setup, and that's your choice of a 20A breaker for the dust collector.  You see, motors draw a significant inrush current on startup; for a 2HP, 115VAC, single phase motor, the breaker will have to handle up to 144A for a short period of time.  So, there's a decent chance that your 20A breaker will trip on overload when you start the dust collector.
Fortunately, the NEC authors are familiar with this issue, and provide an &quot;out&quot; in 430.52(C), which permits you to size inverse-time (read: regular) breakers protecting a single motor load at up to 250% of the full-load current, or up to 45A for a 20A motor load, provided that the motor has its own overload protection.  You probably won't have to go up that high, but having a 25A single-pole breaker for your choice of subpanel handy in case the dust collector won't start on a 20A breaker would be wise (it's the largest value permitted for a 20A motor if the motor doesn't have a built-in overload protector).