Let's take a look at the star and the planet's characteristics first. We have the star TRAPPIST-1:
- $M = 0.082 M_{\odot}$
- $R = 0.117 R_{\odot}$
The planets are all roughly in the scale of $0.7 R_{\oplus}$ to $1.2 R_{\oplus}$ with their semi-major axes of:
- b: $1.66 \times 10^6 \; \mathrm{km}$
- c: $2.88 \times 10^6 \; \mathrm{km}$
- d: $3.14 \times 10^6 \; \mathrm{km}$
- e: $4.19 \times 10^6 \; \mathrm{km}$
- f: $5.57 \times 10^6 \; \mathrm{km}$
- g: $6.73 \times 10^6 \; \mathrm{km}$
So if we would stand on the surface of TRAPPIST-1g and look towards the star, we would be at $1/25$ the distance to the star compared to the sun. The star however is roughly $1/8 R_{\odot}$. That would make the star appear about at about $3$ times the diameter compared to our Sun from the Earth.
TRAPPIST-1f would every few days pass TRAPPIST-1g with a closest approach of $1.16 \times 10^6 \; \mathrm{km} \approx 3 \times r_{\mathrm{Earth-Moon}}$ which is roughly three times the distance to the Moon.
Since $R_{\mathrm{TRAPPIST-1f}} \approx R_{\oplus}$ and $R_{\oplus}/R_{\mathrm{Moon}} = 3.67$ it would seem that TRAPPIST-1f would appear roughly the size of the Moon on closest approach.
The other planets are similar in radius but their distances are $2$ to $5$ times further away and would thus appear smaller by that factor.