¡Era muy bonita! and En el jardín había una piscina muy grande

Hola Nicole

With reference to 1., we are using the imperfect "era" because we are simply "describing"; the speaker has no intention to focus on this being a completed action with an end (where you'd use the Indefinido - fue); It is just descriptive.  

Here is a lesson about this usage of the Imperfecto. 

With reference to 2., the natural order in Spanish is "noun+adjective", unless there is a specific purpose to change it and place the adjective in front. For this there is also a lesson -have a look here. 

Saludos

Inma