There are many house systems for dividing a chart. The default at astro.com is Placidus. And there is the equal house system (each house takes up 30 degrees) that most astrologers doing general sun sign forecast use. For most other house systems, some houses are bigger than others. That is derived from the birth place. There's geometry involved in that which I never dug deep into and don't think is necessary for what I want to be able to do. But I personally use the Koch system. Placidus is pretty popular as well. Basically just pick one that fits your profile - some planets near the cusps of the houses will change houses depending on the house system you use. Find one that fits you and stick with it.

Based on the house system you used to generate the previous chart, your 1st house starts at the ascendant (27 degree Sag) and ends at 1 degree Aquarius. So, Venus at 23 degree Capricorn is in the 1st house.