What commands do I use to make the graphic more presentable, like throwing it to the bottom and increasing the size of the balls?

Uploading: gb.jpeg...

plot<- ggplot(df2, aes(x=V1, y=V2, size = Microplastico_ingerido, label = cod, color = Guild))+
  geom_point(alpha=0.5) +
  xlim(-0.7,0.4)+
  ylim(-0.3,0.6)+
  scale_size(range = c(.2, 24), name="Abundance of Microplastic")+
  geom_text(size = 4.4, colour = "black", vjust = 0, hjust = 0)+
  xlab("Pcoa1 36.09%") +
  ylab("Pcoa2 16.07%") 

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I believe you ask,

What commands do I use to make the graphic more presentable, like throwing it to the bottom and increasing the size of the balls?

Check out the docs site for ggplot2. For point sizes, you want to adjust the size argument, either in aes if you want attach a variable to it (e.g. point sizes are connected to some statistic) or outside to apply a fixed size to all points.

There are a lot of customization options! A good place to get started diving into all that's possible is with R4DS's chapter on ggplots, https://r4ds.had.co.nz/data-visualisation.html

Check out the ggplot2 cheatsheet at https://raw.githubusercontent.com/rstudio/cheatsheets/main/translations/spanish/data-visualization_es.pdf

Can you paste part of your dataset using dput(head(yourdataframe, 15)) to check your code?