IMO, BLT Studio is just the right size for 2 adults...maybe 2 adults and a younger child. Any more than that, and it will feel crowded.
The BLT Studios are small...comparable in size to the Value studios at AKV. If the AKV Value studios feel small/cramped to you, then the BLT studios will also feel small/cramped. If that is the most important thing to you, then i'd probably avoid the BLT studio. IMO, the studio is a fine size, especially if the couch isn't unfolded in to a bed.
The studio at the Poly is definitely bigger. BLT studio is one of the smallest and Poly is one of the biggest. If space is one of the more important features, then BLT studio is probably not a good fit for you.
For me, I don't find the small space at BLT to be a problem. It's small, but very functional. IMO, BLT simply has the best location for getting around all of WDW: Walking Distance to MK. Monorail to MK. Monorail to Epcot (normally, not right now during COVID) Monorail to Poly and GF Restaurants. Boat to Fort Wilderness and Wilderness Lodge restaurants (again, normally...maybe not during COVID). Walk to MK bus stop to get to ALL other hotels/Restaurants at WDW.
Additionally, I enjoy the small footprint of the resort. It's a quick elevator ride and a short walk to any of the hotel amenities (quick service, mug refill, pool, restaurants, etc.) Some other
DVC can be quite the trek to get to the main resort areas (SSR, OKW, etc...).
Again, it VERY much comes down to a matter of what's important for you and your group. For me, I would prefer convenience and amenities over space. You may feel differently. If a small space will make you unhappy, then I would avoid BLT studio.
On the other hand...BLT 1-bedroom is VERY large with tons of room to spread out. Maybe a splurge?